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by everforward
595 days ago
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It depends on how many services connect directly to the database. More than one and there be dragons. The biggest problem is schema changes, because they require the library to be changed. The apps now need to match their library versions against a schema they don’t control. It’s messy to schedule and rollbacks suck really bad. A microservice that returns data from the database can paper over a lot of schema changes, because the responses aren’t directly tied to the schema. Data can be shared without clients ever knowing, entities can be moved and split between tables without clients knowing, etc. I try hard to only have 1 service connect to a database directly. If there’s only 1 app, it can have a direct connection. If there’s more than one, I like to chuck a micro service in front of it. |
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This is only true if multiple applications are accessing the same tables, and even then, only if the changes aren’t backwards-compatible. For example, adding a column shouldn’t be a breaking change, because no one should be doing SELECT * (and also then assuming a fixed width when parsing).
I think another point of common confusion is how overloaded the word “schema” is. Database schema, i.e. how tables are logically related? Table schema? A synonym for database, as MySQL has it? A logical grouping of tables, procedures, etc. within a database, as Postgres has it?
It’s entirely possible (and generally much cheaper, and with less maintenance overhead) for multiple services to share a database cluster/server/node, but have complete separation from one another, modulo indirect impact via the buffer pool.