- Provides a high-availability (HA) solution for directory writes (replicas handle reads)
- As long as one Master is operational, writes can safely be excepted
- Master nodes replicate from each other, so they are always up to date and can be ready to take over (hot standby)
- Syncrepl also allows the master nodes to re-synchronize after any downtime
- Delta-Syncrepl can be used
- MirrorMode is not what is termed as a Multi-Master solution. This is because writes have to go to one of the mirror nodes at a time
- MirrorMode can be termed as Active-Active Hot-Standby, therefor an external server (slapd in proxy mode) or device (hardware load balancer) to manage which master is currently active
- While syncrepl can recover from a completely empty database, slapadd is much faster
- Does not provide faster or more scalable write performance (neither could any Multi-Master solution)
- Backups are managed slightly differently
- If backing up the Berkeley database itself and periodically backing up the transaction log files, then the same member of the mirror pair needs to be used to collect logfiles until the next database backup is taken
- To ensure that both databases are consistent, each database might have to be put in read-only mode while performing a slapcat.
- When using slapcat, the generated LDIF files can be rather large. This can happen with a non-MirrorMode deployment also.
MirrorMode configuration is actually very easy. If you have ever setup a normal slapd syncrepl provider, then the only change is the directive:
mirrormode on
You also need to make you the rid of each mirror node pair is different and that the provider syncrepl directive points to the other mirror pair.
This is the same as the Set up the provider slapd section, referencing delta-syncrepl replication if using delta-syncrepl.
Here's a specific cut down example using LDAP Sync Replication in refreshAndPersist mode (delta-syncrepl can be used also):
MirrorMode node 1:
# syncrepl directives
syncrepl rid=1
provider=ldap://ldap-rid2.example.com
bindmethod=simple
binddn="cn=mirrormode,dc=example,dc=com"
credentials=mirrormode
searchbase="dc=example,dc=com"
schemachecking=on
type=refreshAndPersist
retry="60 +"
mirrormode on
MirrorMode node 2:
# syncrepl directives
syncrepl rid=2
provider=ldap://ldap-rid1.example.com
bindmethod=simple
binddn="cn=mirrormode,dc=example,dc=com"
credentials=mirrormode
searchbase="dc=example,dc=com"
schemachecking=on
type=refreshAndPersist
retry="60 +"
mirrormode on
It's simple really; each MirrorMode node is setup exactly the same, except that the provider directive is set to point to the other MirrorMode node.
There are generally 2 choices for this; 1. Hardware proxies/load-balancing or dedicated proxy software, 2. using a Back-LDAP proxy as a syncrepl provider
MORE HERE and a nice PICTURE
This is exactly the same as the Set up the consumer slapd section. It can either setup in normal syncrepl replication mode, or in delta-syncrepl replication mode.
Hopefully you will now have a directory architecture that provides all of the consistency guarantees of single-master replication, whilst also providing the high availability of multi-master replication.
