E.4. Status information - Appendix E. HA cluster

E.4.1. 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 gaining failover capabilities for the deployment. The most common way to achieve this is to place a load balancer in front of the HA cluster. For a description of HAProxy, see Section E.2.4, “HAProxy for load balancing”. As you can see in that section, it makes use of an HTTP endpoint to discover which instance is the master, and then directs write load to it.

E.4.2. The endpoints

Each HA instance comes with three endpoints regarding its status:

/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 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. The table below outlines the possible responses for each endpoint:

Table E.1. HA HTTP 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`

E.4.3. 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

Unknown status

	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), or the instance could be an arbiter instance. 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.

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), or the instance could be an arbiter instance. 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.

If authentication and authorization is enabled, the HA status endpoints will also require authentication credentials. For some load balancers and proxy servers, providing this with the request is not an option. For those situations, consider disabling authentication of the HA status endpoints by setting dbms.security.ha_status_auth_enabled=false in the neo4j.conf configuration file.