github.com/kardianos/nomad@v0.1.3-0.20151022182107-b13df73ee850/website/source/docs/http/index.html.md (about) 1 --- 2 layout: "http" 3 page_title: "HTTP API" 4 sidebar_current: "docs-http-overview" 5 description: |- 6 Nomad has an HTTP API that can be used to programmatically use Nomad. 7 --- 8 9 # HTTP API 10 11 The Nomad HTTP API is the primary interface to using Nomad, and is used 12 to query the current state of the system as well as to modify it. 13 The Nomad CLI makes use of the Go HTTP client and invokes the HTTP API. 14 15 All API routes are prefixed with `/v1/`. This documentation is only for the v1 API. 16 17 ## Data Model and API Layout 18 19 There are four primary "nouns" in Nomad, these are jobs, nodes, allocations, and evaluations: 20 21 [![Nomad Data Model](/assets/images/nomad-data-model.png)](/assets/images/nomad-data-model.png) 22 23 Jobs are submitted by users and represent a _desired state_. A job is a declarative description 24 of tasks to run which are bounded by constraints and require resources. Nodes are the servers 25 in the clusters that tasks can be scheduled on. The mapping of tasks in a job to nodes is done 26 using allocations. An allocation is used to declare that a set of tasks in a job should be run 27 on a particular node. Scheduling is the process of determining the appropriate allocations and 28 is done as part of an evaluation. 29 30 The API is modeled closely on the underlying data model. Use the links to the left for 31 documentation about specific endpoints. There are also "Agent" APIs which interact with 32 a specific agent and not the broader cluster used for administration. 33 34 ## Blocking Queries 35 36 Certain endpoints support a feature called a "blocking query." A blocking query 37 is used to wait for a potential change using long polling. 38 39 Not all endpoints support blocking, but those that do are clearly designated in the 40 documentation. Any endpoint that supports blocking will also set the HTTP header 41 `X-Nomad-Index`, a unique identifier representing the current state of the 42 requested resource. On subsequent requests for this resource, the client can set the `index` 43 query string parameter to the value of `X-Nomad-Index`, indicating that the client wishes 44 to wait for any changes subsequent to that index. 45 46 In addition to `index`, endpoints that support blocking will also honor a `wait` 47 parameter specifying a maximum duration for the blocking request. This is limited to 48 10 minutes. If not set, the wait time defaults to 5 minutes. This value can be specified 49 in the form of "10s" or "5m" (i.e., 10 seconds or 5 minutes, respectively). 50 51 A critical note is that the return of a blocking request is **no guarantee** of a change. It 52 is possible that the timeout was reached or that there was an idempotent write that does 53 not affect the result of the query. 54 55 ## Consistency Modes 56 57 Most of the read query endpoints support multiple levels of consistency. Since no policy will 58 suit all clients' needs, these consistency modes allow the user to have the ultimate say in 59 how to balance the trade-offs inherent in a distributed system. 60 61 The two read modes are: 62 63 * default - If not specified, the default is strongly consistent in almost all cases. However, 64 there is a small window in which a new leader may be elected during which the old leader may 65 service stale values. The trade-off is fast reads but potentially stale values. The condition 66 resulting in stale reads is hard to trigger, and most clients should not need to worry about 67 this case. Also, note that this race condition only applies to reads, not writes. 68 69 * stale - This mode allows any server to service the read regardless of whether 70 it is the leader. This means reads can be arbitrarily stale; however, results are generally 71 consistent to within 50 milliseconds of the leader. The trade-off is very fast and 72 scalable reads with a higher likelihood of stale values. Since this mode allows reads without 73 a leader, a cluster that is unavailable will still be able to respond to queries. 74 75 To switch these modes, use the `stale` query parameter on request. 76 77 To support bounding the acceptable staleness of data, responses provide the `X-Nomad-LastContact` 78 header containing the time in milliseconds that a server was last contacted by the leader node. 79 The `X-Nomad-KnownLeader` header also indicates if there is a known leader. These can be used 80 by clients to gauge the staleness of a result and take appropriate action. 81 82 ## Cross-Region Requests 83 84 By default any request to the HTTP API is assumed to pertain to the region of the machine 85 servicing the request. A target region can be explicitly specified with the `region` query 86 parameter. The request will be transparently forwarded and serviced by a server in the 87 appropriate region. 88 89 ## Formatted JSON Output 90 91 By default, the output of all HTTP API requests is minimized JSON. If the client passes `pretty` 92 on the query string, formatted JSON will be returned. 93 94 95