decred.org/dcrdex@v1.0.3/spec/orders.mediawiki

=Client Order Management=

__TOC__

This section describes the steps required of the client to place an order, and
the interactions between the client and server to execute an order once a match
has been found.
See the [[atomic.mediawiki/#Atomic_Settlement|atomic settlement]] section for a high-level
overview of the settlement process.

There are three available types of order.

* [[#limit-order|Limit orders]] are used to buy or sell a specified amount of an asset at a rate no higher (buy) or lower (sell) than a specified price.
* [[#market-order|Market orders]] are used to buy or sell a specified amount of an asset at the best price available.
* [[#cancel-order|Cancel orders]] are used to remove standing limit orders from the order book.

The order book holds only limit orders with time in force ''standing'' that have
not completely filled or been canceled. All other orders are only valid for one
[[fundamentals.mediawiki/#epoch-based-order-matching|epoch match cycle]].

==Connection Persistence==

Regardless of connection status, if a client does not respond to their
<code>preimage</code> request, they are in violation of
[[community.mediawiki/#rule-1-clients-must-respond-to-all-preimage-requests|rule 1]] and subject to
penalty.

If a client's connection is lost during match negotiation, the client is
expected to reconnect and complete settlement.
Once a match is made, a client is always subject to violation of
[[community.mediawiki/#rule-2-every-match-must-be-fully-settled|rule 2]] via the
[[fundamentals.mediawiki/#global-variables|broadcast timeout]].

==Order Book Subscriptions==

An order book can be viewed and tracked by subscribing to a market.

'''Request route:''' <code>orderbook</code>, '''originator: ''' client

<code>payload</code>
{|
! field !! type   !! description
|-
| base  || int || the base asset ID
|-
| quote || int || the quote asset ID
|}

The response will contain the complete market order book.
The order book and all updates include a '''sequence ID''', which increments by
+1 whenever the DEX accepts, removes, or modifies an order.
The client is responsible for tracking the sequence ID to ensure all order
updates are received. If an update appears to be missing, the client should
re-subscribe to the market to synchronize the order book from scratch.

'''Response'''

<code>payload</code>
{|
! field    !! type !! description
|-
| marketid || string || A unique market identifier, the market ID.
|-
| seq      || int    || A sequence ID
|-
| epoch    || int  || the current epoch
|-
| orders   || &#91;object&#93; || A list of '''Order''' objects (described below)
|}

'''JSON Order object'''

{|
! field   !! type   !! description !! notes
|-
| seq      || int   || A sequence ID || superceded in <code>orderbook</code> response
|-
| marketid || string || the market ID ||
|-
| oid     || string || the order ID ||
|-
| side    || string || "b" for ''buy'', "s" for ''sell'' || epoch_order, book_order
|-
| qty     || int      || order size (atoms) || epoch_order, book_order
|-
| rate    || int    || price rate. [[comm.mediawiki/#rate-encoding|message-rate encoding]] || only set on limit orders
|-
| tif     || string || time in force. one of "i" for ''immediate'' or "s" for ''standing'' || only set on limit orders
|-
| time    || int    || the order's UNIX timestamp || epoch_order, book_order
|-
| com     || string || the order commitment || epoch_order only
|-
| otype   || string || "l" for ''limit'', "m" for ''market'', "c" for ''cancel'' || epoch_order only
|-
| epoch   || int    || the order's epoch index || epoch_order only
|-
| target  || string || target order ID || only set on cancel orders (epoch_order only)
|}

'''Changes to the order book''' will be received from the DEX as a stream of
notifications. The action to be taken depends on the route according to the
following table. The payload for all three routes is an '''Order''' object.
{|
! route        !! action
|-
| book_order   || add (or update) the order to the order book
|-
| epoch_order  || add the order to the epoch queue
|-
| unbook_order || remove the order from the order book
|}

Additionally, there is a notification for updating the remaining quantity of an
order. This notification is sent when a booked order partially fills and remains
on the book with a reduced matchable quantity.

'''Notification route:''' <code>update_remaining</code>, '''originator: ''' DEX

<code>payload</code>
{|
! field   !! type   !! description
|-
| seq       || int   || A sequence ID
|-
| marketid  || string || The market identifier
|-
| oid       || string || The order ID
|-
| remaining || int    || remaining quantity (atoms)
|}

At the beginning of the matching cycle, the DEX will publish a list of order
preimages, the seed hash used for
[[fundamentals.mediawiki/#pseudorandom-order-matching|order sequencing]], and the
[[#preimage-reveal|commitment checksum]], which together can be used to
independently verify matching.

'''Notification route:''' <code>match_proof</code>, '''originator: ''' DEX

<code>payload</code>
{|
! field     !! type !! description
|-
| marketid  || string || the market ID
|-
| epoch     || int || the epoch index for which the cycle occurs
|-
| preimages || &#91;string&#93; || list of order preimages for the epoch
|-
| misses    || &#91;string&#93; || list of order IDs for which preimages were not received, so were not included in sorting or matching
|-
| csum      || string || the commitment checksum
|-
| seed      || string || epoch queue shuffling seed
|}

A client can '''unsubscribe''' from order book updates without closing the
WebSocket connection.

'''Request route:''' <code>unsub_orderbook</code>, '''originator: ''' client

<code>payload</code>
{|
! field  !! type !! description
|-
| marketid || string || the market ID
|}

<code>result</code>: boolean <code>true</code> on success.

==Order Preparation==

As part of the order, the client must demonstrate control of funds.
This is accomplished by supplying information and a signature for each
[[comm.mediawiki/#coin-id|coin]] that will be spent.
The client covers the ''backing fees'' associated with the inputs spending their
own coins.

In addition, the client must show the ability to cover ''base fees'' for any
initialization transactions that will be generated. The client must show that
they have the funds to cover all fees for the worst-case scenario, which is
single-lot matches for the entire order.
In practice, large orders will rarely pay the total of the base fees because
many of the matches will be more than a single-lot.

===Calculating Transaction Fees===

The '''base fees''' cover transaction fees associated with making
initialization transactions for every match in the order.

For asset '''Z''', a ''base fee ratio'', '''''R<sub>z</sub>''''' is calculated
based on the ''lot size'', '''''l''''' (units ''Z''), a ''fee rate'',
'''''r''''' (''Z/byte''), and a ''transaction size'', '''''s''''' (''bytes'').
'''''s''''' is pre-calculated based on a standard initialization transaction.

<!--R_z = \frac{ s r }{ l }-->
[[File:images/fee_max_ratio.png]]

The ''base fee ratio'' is a constant until the DEX operator changes one of its
factors.

The ''base fees'', '''''f<sub>base</sub>''''' (units ''Z'') can be calculated
from '''''R<sub>z</sub>''''' and the ''order quantity'', '''''Q'''''.

<!--f_{base} = Q R_z-->
[[File:images/base_fees.png]]

The base fees scale linearly with order size, but the actual realized portion of
the base fees, '''''f<sub>fin</sub>''''', can only be known to fall within a
range '''''r s &#8804; f<sub>fin</sub> &#8804; f<sub>base</sub> '''''.

The client also covers the '''backing fees''' associated with spending their
backing coins, '''''f<sub>coin</sub>'''''.
The client must know how to calculate the script sizes to assess fees.
The DEX will verify the coin sum before accepting the order.

===Coin Preparation===

With the exception of market buy orders, which are detailed below, for an order
of quantity '''''Q''''', the sum value of the selected coins, '''''V''''',
must satisfy the relation
(with [[#calculating-transaction-fees|fees]])

<!--V \ge Q + f_{base} + f_{coin}-->
[[File:images/coin-sum.png]]

There may be types of coins which are not supported by the asset's DEX
implementation. Asset developers should make coin-spending limitations clear
to wallet users.

As part of the order, the client will submit a list of ''Coin objects''.

'''JSON Coin object'''

{|
! field     !! type   !! description
|-
| coinid    || string || hex-encoded coin ID
|-
| pubkeys   || &#91;string&#93; || array of hex-encoded pubkeys which spend the coin
|-
| sigs      || &#91;string&#93; || array of signatures of Blake-256 hashes of the serialized coin IDs
|-
| redeem    || string || hex-encoded redeem script for P2SH. empty for P2PKH
|}

In order to enable multi-signature support, more than one pubkey can be
submitted. If more than one pubkey is submitted, there should be a signature
for each one.
The data is signed with the private key(s) corresponding to the
<code>pubkeys</code>.
The <code>pubkeys</code> themselves must correspond with addresses payable by
the coin's pubkey script (or corresponding redeem script).

===Order Commitment===

As part of every submitted order, the client should submit a cryptographic
'''commitment'''.
To generate a commitment, the client creates a random 32-byte sequence,
the ''preimage''. The commitment is the Blake-256 hash of the
preimage. Every order must be assigned a unique commitment, therefore preimages
cannot be reused. They should be generated with a cryptographically-secure
pseudo-random number generator.

The server will reject any zero-valued commitment as well as the specific
commitment generated from hashing a zero-valued preimage. The server can also
reject a commitment that has already been used for any previous order. Depending
on server policy, the historical commitment duplicate search may be limited to
only recent commitments and/or commitments received for orders on a specific
market. Assuming that a cryptographically-secure PRNG is used to generate
preimages, rejection should realistically be impossible.

At the expiration of the epoch, the server sends a <code>preimage</code> request
for each order that is eligible for matching.
The client responds with their preimage(s). If the client fails to respond to
their <code>preimage</code> requests, or if their <code>preimage</code> response
does not hash to their order commitment, the order is not matched and the client
is considered in violation of
[[community.mediawiki/#rule-1-clients-must-respond-to-all-preimage-requests|rule 1]].

The preimages are used as the inputs to
[[fundamentals.mediawiki/#pseudorandom-order-matching|the shuffling algorithm]] to determine
matching order. Before matching commences, the preimages are broadcast
in the <code>match_proof</code> message.

===Order Signing===

All orders must be signed by the client and the server.
The basic signing procedure will involve serializing order data into a byte array
following a specific procedure that can be replicated on the server.
The serialized data is then signed using the client's
[[fundamentals.mediawiki/#identities-based-on-public-key-infrastructure-pki-key-pairs|private account key]].

All integer encoding for all serialized structures is big endian.

All order serializations have common '''prefix''' fields.

'''Prefix fields and serialization'''
{|
! field      !! size (bytes) !! JSON type !! description
|-
| accountid  || 32 || string || hex-encoded client account ID
|-
| base       || 4  || int || the base asset ID
|-
| quote      || 4  || int || the quote asset ID
|-
| ordertype  || 1  || int || the type of order. limit = 1, market=2, cancel=3
|-
| tclient    || 8  || int || the client's UNIX timestamp (milliseconds)
|-
| tserver    || 8  || int || the server's UNIX timestamp (milliseconds). zero for client signature
|-
| com || 32 || string || hex-encoded cryptographic commitment
|}

===Order ID===

The order serialization is used to create a unique order ID.
The ID is defined as the Blake-256 hash of the serialized order, including the
non-zero server's timestamp. The client does not know the order ID when
submitting, but should independently verify the ID after parsing the server's
response.

Because the order ID includes the server's timestamp, the order ID itself
provides a checksum to ensure that order information is properly transmitted.
The response to all order submissions is an '''order receipt''', which includes
the timestamp.

'''Order receipt'''

<code>result</code>
{|
! field     !! type   !! description
|-
| sig       || string || server hex-encoded signature of the serialized order, after adding the DEX timestamp
|-
| orderid   || string || the order ID
|-
| tserver   || int  || the server's UNIX timestamp (milliseconds)
|}

The client should use the server's timestamp to create a serialized order and
independently verify the order ID. The serialized order is also the message for
the server's signature.

==Order Types==

===Limit Order===

Limit orders are for the trade of assets at a rate no higher (buy) or lower
(sell) than a specified price.
The client may specify the ''time in force'' of a limit order as one of: (a)
''standing'', which remains on the books until filled or canceled, or (b)
''immediate'', which can complete execution wholly or partially unfilled. As
such, the ''immediate'' option is intended for limit orders with a price that
crosses the spread (i.e. a taker rather than a maker). The
<code>ordersize</code> must be an integer multiple of the asset's
[[fundamentals.mediawiki/#global-variabless|lot size]].

'''Request route:''' <code>limit</code>, '''originator:''' client

<code>payload</code>
{|
! field       !! type   !! description
|-
| colspan="3" align="center" | 9 prefix fields
|-
| side        || int || buy = 1, sell = 2
|-
| ordersize   || int || order size (atoms)
|-
| rate        || int || price rate. [[comm.mediawiki/#rate-encoding|message-rate encoding]]
|-
| timeinforce || int || standing = 1, immediate = 2
|-
| coins       ||  &#91;[[#Coin_Preparation|Coin]]&#93; || array of funding coins
|-
| address     || string || address where the matched client will send funds
|-
| sig         || string || client hex-encoded signature of the serialized order, with tserver = 0
|}

'''Limit order serialization'''

{|
! field      !! size (bytes)  !! description
|-
| prefix     || 99 || [[#order-signing|the order prefix]]
|-
| coin count || 1  || The number of funding coins
|-
| coin data  || coin length x count || [[#coin-preparation|sequence of coin IDs]]
|-
| side       || 1 || 1 for buy, 2 for sell
|-
| quantity   || 8 || quantity to buy or sell (atoms)
|-
| rate       || 8 || price rate. [[comm.mediawiki/#rate-encoding|message-rate encoding]]
|-
| time in force || 1 || 1 for ''standing'', 2 for ''immediate''
|-
| address    || varies || client's receiving address
|}

<code>result</code>
{|
! field     !! type   !! description
|-
| sig       || string || server hex-encoded signature of the serialized order, after adding the DEX timestamp
|-
| server time || int  || the server's UNIX timestamp (milliseconds)
|}

===Market Order===

A market order is an order to buy or sell an asset at the best available
market price. The request payload fields are similar to a limit order, but
without the <code>rate</code> field or <code>timeinforce</code> fields.

Market orders cannot be canceled.
Any portion of the requested quantity that does not match immediately (during
the epoch match cycle) is left unfilled.

'''Request route:''' <code>market</code>, '''originator: ''' client

<code>payload</code>
{|
! field       !! type   !! description
|-
| colspan="3" align="center" | 9 prefix fields
|-
| side        || int || buy = 1, sell = 2
|-
| ordersize   || int || order size (atoms)
|-
| coins       ||  &#91;[[#coin-preparation|Coin]]&#93; || array of funding coins
|-
| address     || string || address where the matched client will send funds
|-
| sig         || string || client hex-encoded signature of the serialized order, with tserver = 0
|}

'''Market order serialization'''

{|
! field      !! size (bytes)  !! description
|-
| prefix     || 99 || [[#order-signing|the order prefix]]
|-
| coin count || 1  || The number of funding coins
|-
| coin data  || coin length x count || [[#coin-preparation|sequence of coin IDs]]
|-
| side       || 1 || 1 for buy, 2 for sell
|-
| quantity   || 8 || quantity to buy or sell (atoms)
|-
| address    || varies || client's receiving address
|}

<code>result</code>
{|
! field     !! type   !! description
|-
| sig       || string || server hex-encoded signature of the order by server, after adding the DEX timestamp
|-
| server time || int  || the server's UNIX timestamp (milliseconds)
|}

====Market Buy Orders====

Market buy orders have a slightly different ruleset than market sell orders or
limit orders.
First, the <code>ordersize</code> is not denominated in the base asset, but in
the quote asset.
As an example, on the DCR/BTC market, where DCR is the base asset, market sell
orders and both types of limit orders' <code>ordersize</code> are quantified in
the base asset, DCR, but the market buy order's <code>ordersize</code> is in BTC.
The order is essentially a statement of "buy as much DCR as you can with this
much BTC".

The <code>ordersize</code> is also not bound to the integral lot size
constraints of other types of orders.

Since the market may move before the order is matched, at the time of submission
it is not known with certainty how many lots will match.
For orders that are nearly 1 lot, it is possible for no matching to occur
because by the time the order is matched it cannot afford a single lot.
The DEX server maintains an interest in ensuring that only valid orders that
can match are accepted, so market buy orders must be handled carefully to make
sure they remain valid.

To reduce the possibility of market buy orders becoming invalid (too small to
match) due to a price increase, the DEX operator sets a ''market buy buffer'',
'''''b<sub>m</sub> > 1''''' for each market.
For a market where the base asset has lot size '''''l''''', and for which there
is a best known standing sell order price rate, '''''r''''', the
<code>ordersize</code>, '''''Q''''', must satisfy the relation
'''''Q > b<sub>m</sub> l r'''''.
If the best rate increases before the order is matched, the order will still
result in a fill as long as the price does not surpass
~'''''b<sub>m</sub> r'''''.
If the ''market buy buffer'' is set too low or the market is particularly
volatile and the price exceeds '''''b<sub>m</sub> r''''', an order that was
accepted but is now too small to match is considered executed but unfilled and
there is no change to the account's
[[community.mediawiki/#rules-of-community-conduct|cancellation statistics]].

===Cancel Order===

Cancel orders remove standing limit orders from the order book.
A client cannot cancel a market order or a limit order with time in force
''immediate''. Further, due to the epoch-based pseudorandom matching process, a
cancel order submitted in the same epoch as it's corresponding limit order has
a 50% chance of being processed before the order it cancels, resulting in an
error.
This is by design and discourages certain types of spoofing.

'''Request route:''' <code>cancel</code>, '''originator:''' client

<code>payload</code>
{|
! field     !! type   !! description
|-
| colspan="3" align="center" | 9 prefix fields
|-
| targetid  || string || hex-encoded order ID
|-
| sig       || string || client hex-encoded signature of the serialized order data. serialization described below
|}

'''Cancel order serialization'''

{|
! field      !! size (bytes)  !! description
|-
| prefix     || 99 || [[#order-signing|the order prefix]]
|-
| orderid    || 32 || the order ID
|}

<code>result</code>
{|
! field   !! type   !! description
|-
| sig     || string || server hex-encoded signature of the serialize order data, after adding the DEX timestamp
|-
| tserver || int    || the server's UNIX timestamp (milliseconds)
|}

==Preimage Reveal==

At the expiration of the epoch, the DEX sends out a <code>preimage</code>
request for each order in the epoch queue. The match cycle begins 5 seconds
after the last <code>preimage</code> request is sent by the server, so clients
must respond before then.

A '''''commitment checksum''''' is included as part of the
<code>preimage</code> request.
The checksum is the Blake-256 hash of the concatenated, lexicographically-sorted
commitments for every order in the epoch. For clients subscribed to the order
book for the entire duration of the epoch, the checksum can be validated against
the checksum generated from their local copy of the epoch queue.

'''Request route:''' <code>preimage</code>, '''originator:''' DEX

<code>payload</code>
{|
! field   !! type   !! description
|-
| orderid || string || order ID
|-
| csum    || string || the commitment checksum
|}

'''Preimage response'''

<code>result</code>
{|
! field   !! size (bytes)  !! description
|-
| pimg    || string || hex-encoded preimage for the order's commitment
|}

==Match negotiation==

Swap negotiation details will be relayed through the DEX with a series of
notifications.
Both the DEX and the clients will need to serialize and sign the notification
data. The originator includes their signature with the request, while the
recipient will return an '''acknowledgement''', or a list of
acknowledgements, as the <code>result</code> of their response payload.

'''Acknowledgement'''

{|
! field     !! type   !! description
|-
| matchid   ||  string  || the match ID
|-
| sig       ||  string || hex-encoded signature of the notification data
|}

If the client's order has one or more matches at the end of a match cycle, the
DEX will send a list of '''match objects'''. The maker is the first to act, so
after sending their acknowledgement, they should broadcast their initialization
transaction and inform the server with an <code>init</code> notification
(described after).

'''Request route:''' <code>match</code>, '''originator:''' DEX

<code>payload</code> (array)
{|
! field     !! type   !! description
|-
| orderid   || string || order ID
|-
| matchid   || string    || the match ID to use for progress notifications
|-
| qty       || int    || the matched amount, in atoms of the base asset
|-
| rate      || int    || the matched price rate. [[comm.mediawiki/#rate-encoding|message-rate encoding]]
|-
| tserver   || int    || server's UNIX timestamp (milliseconds)
|-
| address   || string || the counterparty's receiving address
|-
| side      || int    || the client's side in the match. 0 = maker, 1 = taker
|-
| status    || int    || only provided in 'connect' response. For 'match' requests, status is 0 = 'MakerSwapCast'. See [[https://github.com/decred/dcrdex/blob/master/dex/order/match.go|match.go]] for codes.
|-
| sig       || string || DEX's hex-encoded signature of the serialized notification data. serialization described below
|}

'''Match serialization'''

{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32 || the order ID
|-
| matchid    || 32  || the ID assigned to this match
|-
| quantity   || 8  || the matched amount, in atoms of the base asset
|-
| rate       || 8  || the matched price rate. [[comm.mediawiki/#rate-encoding|message-rate encoding]]
|-
| tserver  || 8  || server's UNIX timestamp (milliseconds)
|-
| address    || varies || UTF-8 encoded receiving address for the match
|}

'''The <code>tserver</code> value is used as the basis for the the locktimes.'''
If it is necessary to convert the time to seconds, the value should be rounded
down.

The client will respond with a list of signed match acknowledgements.

After a client broadcasts their initialization transaction, they are
expected to report the transaction details to the server for verification and
relay to the matching party.

'''Request route:''' <code>init</code>, '''originator:''' client

<code>payload</code>
{|
! field      !! type   !! description
|-
| orderid    || string || the order ID
|-
| matchid    || string    || the matchid, retrieved from the [[#match-negotiation|match notification]]
|-
| coinid    || string || hex-encoded coin ID
|-
| contract   || string || hex-encoded swap redeem script
|-
| sig        || string || client signature of the serialized notification. serialization described below
|}

'''Init serialization'''

{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32 || the order ID
|-
| matchid    || 32  || the ID assigned to this match
|-
| coinid     || asset-dependent  || the coin ID
|-
| contract   || asset-dependent || swap redeem script
|}

The DEX will respond with an acknowledgement.

The DEX will send each client a notification when the counterparty has broadcast
their initialization transaction.
When the taker receives the <code>audit</code> notification, they will audit the
maker's contract and after that contract asset's configured swap confirmation
requirement is reached, broadcast their own initialization.
When the maker receives the <code>audit</code> notification, they will audit the
taker's contract and after the required confirmations, issue their redemption.

'''Request route:''' <code>audit</code>, '''originator:''' DEX

<code>payload</code>
{|
! field     !! type   !! description
|-
| orderid   || string || the order ID
|-
| matchid   || string || the match ID
|-
| timestamp || int  || server's UNIX timestamp (milliseconds)
|-
| coinid    || string || hex-encoded coin ID
|-
| contract  || string || hex-encoded swap redeem script
|-
| sig       || string || DEX's signature of the serialized notification. serialization described below
|}

'''Audit serialization'''

{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32 || the order ID
|-
| matchid    || 32  || the match ID
|-
| timestamp  || 8  || server's UNIX timestamp (milliseconds)
|-
| coin ID    || asset-dependent  || the coin ID
|-
| contract   || asset-dependent || swap redeem script
|}

The client responds with an acknowledgement.

When a client has redeemed their contract, they will notify the server.

'''Request route:''' <code>redeem</code>, '''originator:''' client

<code>payload</code>
{|
! field      !! type   !! description
|-
| orderid    || string || the order ID
|-
| matchid    || string    || the match ID
|-
| coinid     || string || hex-encoded coin ID
|-
| secret     || string || the hex-encoded swap contract secret
|-
| sig        || string || client signature of the serialized notification. serialization described below
|}

'''Redeem serialization'''
{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32 || the order ID
|-
| matchid    || 32  || the match ID
|-
| coin ID    || asset-dependent  || the coin ID
|-
| secret     || 32  || the swap contract secret
|}

The DEX responds with an acknowledgement.

The DEX informs the taker when the maker has redeemed.

'''Request route:''' <code>redemption</code>, '''originator:''' DEX

<code>payload</code>
{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32 || the order ID
|-
| matchid    || 32  || the match ID
|-
| coinid     || string || hex-encoded coin ID
|-
| secret     || string || the hex-encoded swap contract secret
|-
| timestamp  || int    || server's UNIX timestamp (milliseconds)
|-
| sig        || string || DEX's signature of the serialized notification. serialization described below
|}

'''Redemption serialization'''

{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32 || the order ID
|-
| matchid    || 32  || the match ID
|-
| coin ID    || asset-dependent  || the coin ID
|-
| secret     || 32  || the swap contract secret
|-
| timestamp  || 8  || server's UNIX timestamp (milliseconds)
|}

The client will respond with an acknowledgement.

The taker will get the key from the maker's redemption and broadcast their own
redemption transaction.

It is also possible for an epoch order to go through the matching cycle without
generating a match. This will be common for limit orders, but can also occur for
market orders if there are no booked orders to match with. When the server fails
to find any matches, a <code>nomatch</code> notification will be sent to the
client.

'''Notification route:''' <code>nomatch</code>, '''originator:''' DEX

<code>payload</code>
{|
! field   !! type   !! description
|-
| orderid || string || order ID
|}

A client can request the current match status using the
<code>match_status</code> request. Note that the route is provided as a
convenience, and persistence of match data may be subject to the operator's
archiving policy. <code>match_status</code> can be used to recover after a
temporary disconnection, but should not be relied on as a source of historical
match data.

'''Notification route:''' <code>match_status</code>, '''originator:''' client

<code>payload</code>: an array of <code>MatchRequest</code> objects

'''<code>MatchRequest</code> object'''

{|
! field   !! type   !! description
|-
| base    || int    || the base asset ID
|-
| quote   || int    || the quote asset ID
|-
| matchid || string || hex-encoded Match ID
|}

<code>result</code>: an array of <code>MatchStatus</code> objects

'''<code>MatchStatus</code> object'''

{|
! field         !! type   !! description
|-
| matchid       || string || the match ID
|-
| status        || int    || See [[https://github.com/decred/dcrdex/blob/master/dex/order/match.go|match.go]] for codes.
|-
| makercontract || string || the swap contract broadcast by the maker
|-
| takercontract || string || the swap contract broadcast by the taker
|-
| makerswap     || string || the coin ID for the swap broadcast by the maker
|-
| takerswap     || string || the coin ID for the swap broadcast by the taker
|-
| makerredeem   || string || the coin ID for the maker's redemption
|-
| takerredeem   || string || the coin ID for the taker's redemption
|-
| secret        || string || the swap contract secret
|-
| active        || bool   || whether the match is still active
|}

Empty fields may be omitted from the encoded <code>MatchStatus</code> object.
Results are only returned for matches that could be found in the server
records. No error is set in the case of unfound matches.

==Match Revocation==

A match can be revoked by the server if a client fails to act within the
[[fundamentals.mediawiki/#global-variables|broadcast timeout]]. A match revocation will result in
penalties for the violating party only.
The revoked match quantity is not added back to the order book in any form.

'''Request route:''' <code>revoke_match</code>, '''originator:''' DEX

<code>payload</code>
{|
! field    !! type   !! description
|-
| orderid  || string || the order ID
|-
| matchid  ||  string  || the match ID
|-
| sig      || string || DEX's hex-encoded signature of serialized revocation. serialization described below
|}

'''Revocation serialization'''

{|
! field      !! size (bytes)  !! description
|-
| orderid    || 32  || the order ID
|-
| matchid    || 32  || the match ID
|}

The client will respond with an acknowledgement.

==Trade Suspension==

There are a number of scenarios where the server may suspend operations,
intentionally or not.
During trade suspension, standing limit orders are not necessarily revoked.

If the server intentionally suspends operations, they should provide a
notification to connected clients as early as possible, ideally with several
epochs for the client to get their orders situated before matching ceases.

Clients should be prepared to lose connection during suspension. Clients will
need to reconnect and complete settlement when the server comes back online.

If the server disconnects without notice, it is expected that orders placed
during the current epoch are revoked at no penalty to the client and that
standing limit orders are persisted.

The suspension notification may indicate that standing limit orders will not be
persisted.
This would be the case if the DEX needs to change an asset variable such as
the lot size or minimum transaction fee rate.

If standing limit orders are persisted, they will be auto-revoked if the client
does not reconnect before the next [[comm.mediawiki/#session-authentication|start epoch]].

'''Request route: ''' <code>suspension</code>, '''originator:''' DEX

<code>payload</code>
{|
! field       !! type   !! description
|-
| marketid    || string || the market ID
|-
| finalepoch  || uint64 || the last epoch during which orders will be collected and matched
|-
| suspendtime || uint64 || the UNIX timestamp corresponding to the end of the final epoch (milliseconds)
|-
| persistbook || bool   || whether standing limit orders will persist through the suspension
|}

Clients will also be informed when trading is resumed.

'''Request route: ''' <code>resumption</code>, '''originator:''' DEX
<code>payload</code>
{|
! field       !! type   !! description
|-
| marketid    || string || the market ID
|-
| startepoch  || uint64 || the epoch number at which trading did or will commence. May be in the future
|-
| epochlen || uint64 || the [[#epoch-based-order-matching|epoch duration]] (milliseconds)
|}