Month: May 2018

This is the fifth in a series of posts reviewing methods for MySQL master discovery: the means by which an application connects to the master of a replication tree. Moreover, the means by which, upon master failover, it identifies and connects to the newly promoted master.

These posts are not concerned with the manner by which the replication failure detection and recovery take place. I will share orchestrator specific configuration/advice, and point out where cross DC orchestrator/raft setup plays part in discovery itself, but for the most part any recovery tool such as MHA, replication-manager, severalnines or other, is applicable.

We discuss asynchronous (or semi-synchronous) replication, a classic single-master-multiple-replicas setup. A later post will briefly discuss synchronous replication (Galera/XtraDB Cluster/InnoDB Cluster).

Master discovery via Service discovery and Proxy

Part 4 presented with an anti-pattern setup, where a proxy would infer the identify of the master by drawing conclusions from backend server checks. This led to split brains and undesired scenarios. The problem was the loss of context.

We re-introduce a service discovery component (illustrated in part 3), such that:

• The app does not own the discovery, and
• The proxy behaves in an expected and consistent way.

In a failover/service discovery/proxy setup, there is clear ownership of duties:

• The failover tool own the failover itself and the master identity change notification.
• The service discovery component is the source of truth as for the identity of the master of a cluster.
• The proxy routes traffic but does not make routing decisions.
• The app only ever connects to a single target, but should allow for a brief outage while failover takes place.

Depending on the technologies used, we can further achieve:

• Hard cut for connections to old, demoted master M.
• Black/hold off for incoming queries for the duration of failover.

We explain the setup using the following assumptions and scenarios:

• All clients connect to master via cluster1-writer.example.net, which resolves to a proxy box.
• We fail over from master M to promoted replica R.

Continue reading » “MySQL master discovery methods, part 5: Service discovery & Proxy”

Note: the method described here is an anti pattern

This is the fourth in a series of posts reviewing methods for MySQL master discovery: the means by which an application connects to the master of a replication tree. Moreover, the means by which, upon master failover, it identifies and connects to the newly promoted master.

These posts are not concerned with the manner by which the replication failure detection and recovery take place. I will share orchestrator specific configuration/advice, and point out where cross DC orchestrator/raft setup plays part in discovery itself, but for the most part any recovery tool such as MHA, replication-manager, severalnines or other, is applicable.

We discuss asynchronous (or semi-synchronous) replication, a classic single-master-multiple-replicas setup. A later post will briefly discuss synchronous replication (Galera/XtraDB Cluster/InnoDB Cluster).

Master discovery via Proxy Heuristics

In Proxy Heuristics all clients connect to the master through a proxy. The proxy observes the backend MySQL servers and determines who the master is.

This setup is simple and easy, but is an anti pattern. I recommend against using this method, as explained shortly.

Clients are all configured to connect to, say, cluster1-writer.proxy.example.net:3306. The proxy will intercept incoming requests either based on hostname or by port. It is aware of all/some MySQL backend servers in that cluster, and will route traffic to the master M.

A simple heuristic that I’ve seen in use is: pick the server that has read_only=0, a very simple check.

Let’s take a look at how this works and what can go wrong.

Continue reading » “MySQL master discovery methods, part 4: Proxy heuristics”

This is the third in a series of posts reviewing methods for MySQL master discovery: the means by which an application connects to the master of a replication tree. Moreover, the means by which, upon master failover, it identifies and connects to the newly promoted master.

These posts are not concerned with the manner by which the replication failure detection and recovery take place. I will share orchestrator specific configuration/advice, and point out where cross DC orchestrator/raft setup plays part in discovery itself, but for the most part any recovery tool such as MHA, replication-manager, severalnines or other, is applicable.

We discuss asynchronous (or semi-synchronous) replication, a classic single-master-multiple-replicas setup. A later post will briefly discuss synchronous replication (Galera/XtraDB Cluster/InnoDB Cluster).

App & service discovery

Part 1 and part 2 presented solutions where the app remained ingorant of master’s identity. This part takes a complete opposite direction and gives the app ownership on master access.

We introduce a service discovery component. Commonly known are Consul, ZooKeeper, etcd, highly available stores offering key/value (K/V) access, leader election or full blown service discovery & health.

We satisfy ourselves with K/V functionality. A key would be mysql/master/cluster1 and a value would be the master’s hostname/port.

It is the app’s responsibility at all times to fetch the identity of the master of a given cluster by querying the service discovery component, thereby opening connections to the indicated master.

The service discovery component is expected to be up at all times and to contain the identity of the master for any given cluster.

Continue reading » “MySQL master discovery methods, part 3: app & service discovery”

This is the second in a series of posts reviewing methods for MySQL master discovery: the means by which an application connects to the master of a replication tree. Moreover, the means by which, upon master failover, it identifies and connects to the newly promoted master.

These posts are not concerned with the manner by which the replication failure detection and recovery take place. I will share orchestrator specific configuration/advice, and point out where cross DC orchestrator/raft setup plays part in discovery itself, but for the most part any recovery tool such as MHA, replication-manager, severalnines or other, is applicable.

We discuss asynchronous (or semi-synchronous) replication, a classic single-master-multiple-replicas setup. A later post will briefly discuss synchronous replication (Galera/XtraDB Cluster/InnoDB Cluster).

Master discovery via VIP

In part 1 we saw that one the main drawbacks of DNS discovery is the time it takes for the apps to connect to the promoted master. This is the result of both DNS deployment time as well as client’s TTL.

A quicker method is offered: use of VIPs (Virtual IPs). As before, apps would connect to cluster1-writer.example.net, cluster2-writer.example.net, etc. However, these would resolve to specific VIPs.

Say cluster1-writer.example.net resolves to 10.10.0.1. We let this address float between servers. Each server has its own IP (say 10.20.0.XXX) but could also potentially claim the VIP 10.10.0.1.

VIPs can be assigned by switches and I will not dwell into the internals, because I’m not a network expert. However, the following holds:

• Acquiring a VIP is a very quick operation.
• Acquiring a VIP must take place on the acquiring host.
• A host may be unable to acquire a VIP should another host holds the same VIP.
• A VIP can only be assigned within a bounded space: hosts connected to the same switch; hosts in the same Data Center or availability zone.

Continue reading » “MySQL master discovery methods, part 2: VIP & DNS”

This is the first in a series of posts reviewing methods for MySQL master discovery: the means by which an application connects to the master of a replication tree. Moreover, the means by which, upon master failover, it identifies and connects to the newly promoted master.

These posts are not concerned with the manner by which the replication failure detection and recovery take place. I will share orchestrator specific configuration/advice, and point out where cross DC orchestrator/raft setup plays part in discovery itself, but for the most part any recovery tool such as MHA, replication-manager, severalnines or other, is applicable.

We discuss asynchronous (or semi-synchronous) replication, a classic single-master-multiple-replicas setup. A later post will briefly discuss synchronous replication (Galera/XtraDB Cluster/InnoDB Cluster).

Master discovery via DNS

In DNS master discovery applications connect to the master via a name that gets resolved to the master’s box. By way of example, apps would target the masters of different clusters by connecting to cluster1-writer.example.net, cluster2-writer.example.net, etc. It is up for the DNS to resolve those names to IPs.

Continue reading » “MySQL master discovery methods, part 1: DNS”