Every “build a lock on Redis” tutorial spends its time on acquisition: SET key value NX PX 30000, one winner, done. The release gets one line, DEL key. That line is where most of the lock bugs I’ve had in ePHPm lived. Over about three months the same mistake showed up in three places. First the native session handler, then two of the Composer wrapper packages, and finally the cluster. Each time it wore a slightly different costume.

By the end of Part 2 ePHPm v0.7.0 ran exactly one SQLite engine in process. That was Turso, for single-node and clustered mode alike. The sidecar was gone. What I skipped over is how a replica actually gets the primary’s writes, and what happens when the primary dies with data on it. That’s this post.

ePHPm gets its own certificates from Let’s Encrypt. In a cluster only one node should be talking to the CA. The nodes elect an ACME leader over the shared KV store. The leader orders the cert, and everybody else installs it from KV. That all had unit tests and had worked on single nodes for months. Then I tried to move wildcard TLS termination for our preview environment off a separate proxy and onto the 3-node cluster itself. In about two days it found three separate bugs that each took…

Most PHP sites I have run in the last twenty years did not need a MySQL server. They needed a database, and MySQL was the one every framework spoke. With ePHPm I have been trying to get rid of that server for the common case. The piece that makes it possible is litewire. PHP connects with plain pdo_mysql. litewire speaks the MySQL wire protocol, and the queries land in a SQLite file. WordPress and Laravel don’t know the difference. No mysqld, no child processes, just ephpm and a .db file.

Most PHP developers never touch clustering primitives. That’s by design. PHP is share-nothing. Each request starts from zero, and anything shared lives somewhere else. Sessions go in Redis. Cache goes in Redis or Memcached. Locks go in Redis. When you run two app servers, the “cluster” is really two stateless boxes pointed at the same Redis. That works until Redis is the thing you’re trying not to run.