open-consul/website/source/api/txn.html.md

layout	page_title	sidebar_current	description
api	Transaction - HTTP API	api-txn	The /txn endpoints manage updates or fetches of multiple keys inside a single, atomic transaction.

Transactions HTTP API

The /txn endpoints manage updates or fetches of multiple keys inside a single, atomic transaction. It is important to note that each datacenter has its own KV store, and there is no built-in replication between datacenters.

Create Transaction

This endpoint permits submitting a list of operations to apply to the KV store inside of a transaction. If any operation fails, the transaction is rolled back and none of the changes are applied.

If the transaction does not contain any write operations then it will be fast-pathed internally to an endpoint that works like other reads, except that blocking queries are not currently supported. In this mode, you may supply the ?stale or ?consistent query parameters with the request to control consistency. To support bounding the acceptable staleness of data, read-only transaction responses provide the X-Consul-LastContact header containing the time in milliseconds that a server was last contacted by the leader node. The X-Consul-KnownLeader header also indicates if there is a known leader. These won't be present if the transaction contains any write operations, and any consistency query parameters will be ignored, since writes are always managed by the leader via the Raft consensus protocol.

Method	Path	Produces
PUT	/txn	application/json

The table below shows this endpoint's support for blocking queries, consistency modes, agent caching, and required ACLs.

Blocking Queries	Consistency Modes	Agent Caching	ACL Required
NO	all1	none	key:read,key:write2

¹ For read-only transactions
² The ACL required depends on the operations in the transaction.

Parameters

• dc (string: "") - Specifies the datacenter to query. This will default to the datacenter of the agent being queried. This is specified as part of the URL as a query parameter.

• KV is the only available operation type, though other types may be added in the future.

  • Verb (string: <required>) - Specifies the type of operation to perform. Please see the table below for available verbs.

  • Key (string: <required>) - Specifies the full path of the entry.

  • Value (string: "") - Specifies a base64-encoded blob of data. Values cannot be larger than 512kB.

  • Flags (int: 0) - Specifies an opaque unsigned integer that can be attached to each entry. Clients can choose to use this however makes sense for their application.

  • Index (int: 0) - Specifies an index. See the table below for more information.

  • Session (string: "") - Specifies a session. See the table below for more information.

Sample Payload

The body of the request should be a list of operations to perform inside the atomic transaction. Up to 64 operations may be present in a single transaction.

[
  {
    "KV": {
      "Verb": "<verb>",
      "Key": "<key>",
      "Value": "<Base64-encoded blob of data>",
      "Flags": <flags>,
      "Index": <index>,
      "Session": "<session id>"
    }
  }
]

Sample Request

$ curl \
    --request PUT \
    --data @payload.json \
    http://127.0.0.1:8500/v1/txn

Sample Response

If the transaction can be processed, a status code of 200 will be returned if it was successfully applied, or a status code of 409 will be returned if it was rolled back. If either of these status codes are returned, the response will look like this:

{
  "Results": [
    {
      "KV": {
        "LockIndex": <lock index>,
        "Key": "<key>",
        "Flags": <flags>,
        "Value": "<Base64-encoded blob of data, or null>",
        "CreateIndex": <index>,
        "ModifyIndex": <index>
      }
    }
  ],
  "Errors": [
    {
      "OpIndex": <index of failed operation>,
      "What": "<error message for failed operation>"
    },
  ]
}

• Results has entries for some operations if the transaction was successful. To save space, the Value will be null for any Verb other than "get" or "get-tree". Like the /v1/kv/<key> endpoint, Value will be Base64-encoded if it is present. Also, no result entries will be added for verbs that delete keys.

• Errors has entries describing which operations failed if the transaction was rolled back. The OpIndex gives the index of the failed operation in the transaction, and What is a string with an error message about why that operation failed.

Table of Operations

The following table summarizes the available verbs and the fields that apply to that operation ("X" means a field is required and "O" means it is optional):

Verb	Operation	Key	Value	Flags	Index	Session
set	Sets the Key to the given Value	x	x	o
cas	Sets, but with CAS semantics	x	x	o	x
lock	Lock with the given Session	x	x	o		x
unlock	Unlock with the given Session	x	x	o		x
get	Get the key, fails if it does not exist	x
get-tree	Gets all keys with the prefix	x
check-index	Fail if modify index != index	x			x
check-session	Fail if not locked by session	x				x
check-not-exists	Fail if key exists	x
delete	Delete the key	x
delete-tree	Delete all keys with a prefix	x
delete-cas	Delete, but with CAS semantics	x			x