> In any case, I believe a DB per backend service isn't a decision driven by the frontend - rather, it's driven by data migration and data access requirements.

I think the idea of breaking up a shared enterprise DB into many distinct but communicating and dependent DB's was driven by a desire to reduce team+system dependencies to increase ability to change.

While the pro is valid and we make use of the idea sometimes when we design things, the cons are significant. Splitting up a DB that has data that is naturally shared by many departments in the business and by many modules/functional areas of the system increases complexity substantially.

In the shared model, when some critical attribute of an item (sku) is updated, then all of the different modules+functional areas of enterprise are immediately using that current and correct master value.

In the distributed model, there is significant complexity and effort to share this state across all areas. I've worked on systems designed this way and this issue frequently causes problems related to timing.

As with everything, no single solution is best for all situations. We only split this kind of shared state when the pros outweigh the cons, which is sometimes but not that often.

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mdavid626 5 days ago | parent [-]

I disagree. I generally understand the problem a "split-up" database brings to the table. This is how people designed things in the last many decades.

What I propose is to leave this design behind.

The split up design fits modern use cases much better. People want all kind of data. They want to change what data they want rather often.

"One" database for all of this doesn't really work -- you can't change the schema since it's used by many applications. So, you'll stuck with a design coming from a time when requirements were probably quite different. Of course, you can make some modifications, but not many and not fundamental ones.

In the split-up design, since you're not sharing the database, you can do whatever you want. Change schema as you see fit. Store data in multiple different forms (duplicates), so it can be queried quickly. The only thing you have to keep is the interface to the outside world (department etc.). Here you can use eg. versioning of your API. Handy.

The 90's are over. We don't have to stick to the limitations people had back then.

Yes of course, data not being up-to-date in every system can be a problem. BUT business people nowadays tend to accept that more, than the inability to change data structures ("we can't add a new field", "we can't change this field" etc.).

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RaftPeople 4 days ago | parent | next [-]

> In the split-up design, since you're not sharing the database, you can do whatever you want.

> we can't add a new field, we can't change this field

Ok, let's do an example.

Assumption:

A-ERP system with approximately 30 modules in use (e.g. sales order mgmt, inventory, purchasing, etc)

B-For split DB, the DB is split by module and data flows exist for all shared data. So there are X different copies of the item master (many and possibly most of those modules use the item master), each with the subset of data required by the specific module.

Sample change, add a new field to the item master:

Shared DB:

1-Update DB schema for item master

2-Update code in different modules that need to use the new data element (per feature requirements)

Split DB:

1-Update DB schema in all modules that require the new data element (per feature requirements)

2-Update code in different modules that need to use the new data element

3-Update the data flows for item data in each module that needs to use the new data element

I think you're understating the level of effort when you say "now we can do whatever we want". The actual effort in this change (which is a very common example) is actually greater than in a shared DB and requires more coordination.

Again, there are times when it's the right thing to do, but definitely not a silver bullet without trade-offs.

	▲	mdavid626 2 days ago \| parent [-]
		It's interesting to see what people consider difficult to do. In my opinion the "Split DB" case you outlined is still much easier to do. It's never the lines of code or number of steps that need to be done make it complicated or difficult. It's always the strange, weird, unexpected things. I change "this" and "that" breaks, but nobody knows why. The biggest benefit of my approach is that it can be split-up between people. One team handles this part, the other some other part. You can only break your part of the database, not everything for everyone else.

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sgarland 5 days ago | parent | prev [-]

If you have to change your schema frequently, you didn’t adequately (or at all, more likely) model your data.

DB schema is supposed to be inflexible and strict; that’s how you can guarantee that the data it’s storing is correct.

> The 90s are over

And now we have a generation of devs who think that 1 msec latency for disk reads is normal, that applications need to ship their own OS to run, and that SQL is an antiquated language that they don’t need to bother to learn.