In-database sharded queue for a shared-disk database

A non-transitory computer-readable medium carrying one or more sequences of instructions which, when executed by one or more processors, cause the one or more processors to carry out the steps of:

• storing first messages from at least one first enqueuer on a first shard of a queue, the at least one first enqueuer executing in a first instance of a shared-disk database system, the first shard comprising a first plurality of subshards, wherein each first message is stored on one of the first plurality of subshards;
• storing second messages from at least one second enqueuer on a second shard of the queue, the at least one second enqueuer executing in a second instance of the shared-disk database system, the second shard comprising a second plurality of subshards, wherein each second message is stored on one of the second plurality of subshards;
• maintaining, in volatile memory of the first instance, a first message cache for the first shard, the first message cache comprising a first buffer set comprising at least one buffer associated with each of the first plurality of subshards;
• maintaining, in volatile memory of the second instance, a second message cache for the second shard, the second message cache comprising a second buffer set comprising at least one buffer associated with each of the second plurality of subshards;
• wherein storing a particular message on a particular subshard comprises storing an entry for the particular message in the buffer associated with the particular subshard;
• when a first subscriber is configured to dequeue messages from the at least one first enqueuer, causing a first dequeue session of a first subscriber executing on the first instance to dequeue first messages from the first message cache;
• when the first subscriber is configured to dequeue messages from the at least one second enqueuer, causing a second dequeue session of the first subscriber to dequeue second messages from the second message cache.