26.7. REST endpoint for HA status information

Introduction

A common use case for Neo4j HA clusters is to direct all write requests to the master while using slaves for read operations, distributing the read load across the cluster and and gain failover capabilities for your deployment. The most common way to achieve this is to place a load balancer in front of the HA cluster, an example being shown with HA Proxy. As you can see in that guide, it makes use of a REST endpoint to discover which instance is the master and direct write load to it. In this section, we’ll deal with this REST endpoint and explain its semantics.

The endpoints

Each HA instance comes with 3 endpoints regarding its HA status. They are complimentary but each may be used depending on your load balancing needs and your production setup. Those are:

  • /db/manage/server/ha/master
  • /db/manage/server/ha/slave
  • /db/manage/server/ha/available

The /master and /slave endpoints can be used to direct write and non-write traffic respectively to specific instances. This is the optimal way to take advantage of Neo4j’s scaling characteristics. The /available endpoint exists for the general case of directing arbitrary request types to instances that are available for transaction processing.

To use the endpoints, perform an HTTP GET operation on either and the following will be returned:

HA REST endpoint responses

Endpoint Instance State Returned Code Body text

/db/manage/server/ha/master

Master

200 OK

true

Slave

404 Not Found

false

Unknown

404 Not Found

UNKNOWN

/db/manage/server/ha/slave

Master

404 Not Found

false

Slave

200 OK

true

Unknown

404 Not Found

UNKNOWN

/db/manage/server/ha/available

Master

200 OK

master

Slave

200 OK

slave

Unknown

404 Not Found

UNKNOWN

Examples

From the command line, a common way to ask those endpoints is to use curl. With no arguments, curl will do an HTTP GET on the URI provided and will output the body text, if any. If you also want to get the response code, just add the -v flag for verbose output. Here are some examples:

  • Requesting master endpoint on a running master with verbose output
#> curl -v localhost:7474/db/manage/server/ha/master
* About to connect() to localhost port 7474 (#0)
*   Trying ::1...
* connected
* Connected to localhost (::1) port 7474 (#0)
> GET /db/manage/server/ha/master HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: localhost:7474
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: text/plain
< Access-Control-Allow-Origin: *
< Transfer-Encoding: chunked
< Server: Jetty(6.1.25)
<
* Connection #0 to host localhost left intact
true* Closing connection #0
  • Requesting slave endpoint on a running master without verbose output:
#> curl localhost:7474/db/manage/server/ha/slave
false
  • Finally, requesting the master endpoint on a slave with verbose output
#> curl -v localhost:7475/db/manage/server/ha/master
* About to connect() to localhost port 7475 (#0)
*   Trying ::1...
* connected
* Connected to localhost (::1) port 7475 (#0)
> GET /db/manage/server/ha/master HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: localhost:7475
> Accept: */*
>
< HTTP/1.1 404 Not Found
< Content-Type: text/plain
< Access-Control-Allow-Origin: *
< Transfer-Encoding: chunked
< Server: Jetty(6.1.25)
<
* Connection #0 to host localhost left intact
false* Closing connection #0
[Note]Unknown status

The UNKNOWN status exists to describe when a Neo4j instance is neither master nor slave. For example, the instance could be transitioning between states (master to slave in a recovery scenario or slave being promoted to master in the event of failure). If the UNKNOWN status is returned, the client should not treat the instance as a master or a slave and should instead pick another instance in the cluster to use, wait for the instance to transit from the UNKNOWN state, or undertake restorative action via systems admin.