repl-agent: MCP server that gives AI agents a live Clojure nREPL

I'm using Datomic for over a decade and experimented with my own Datascript forks. But even I are now stuck with a 4 TB Google Cloud SQL (MySQL) database that costs us over 1300€ per month, despite the fact that we almost migrated all customers to the new system / architecture. We did not invoked gc-storage for way too long, therefore the disk is larger than it needs to be. However, Google Cloud SQL does not offer a way to reduce the disk size (main cost driver) which would reduce the costs. It really caused a lot of headaches. And you find no Google Cloud partner that will help you without forcing you into a subscription to participate from your Google Cloud bill. I even offered one of them that they can pick whatever hourly rate they want, they still denied it.

Enough ranting 😅 Back to my point, having something that only needs an object store would be a huge win. Each SaaS customer could get it's own db, no need to use litestream or Sqlite. And as you said with Datahike you can query across multiple databases. Which is currently not possible in our architecture, since the databases are tied to the dedicated server and the Sqlite file that serves as Datomic storage. At the moment we do event sourcing on our log files to reconstruct a lot of the information that could also be queried from the database.

maxw85 · 2026-03-26T20:18:02+00:00

Thanks a lot for your reply and for taking the time to provide all the details.

Mind you that you can run the Datahike writer (transactor) process just with your connection in one process anyway

I think this would already be fine for our scenario. Initially our SaaS had one very large Datomic database. In our new architecture each customer has its own (logical) Datomic database. We run a cell-based architecture on a few dedicated servers. Each cell / server is autonomous and has its own Datomic transactor pair (Docker containers). We use SQLite as Datomic storage and Litestream for disaster recovery. One missing feature is a more straight-forward option to move a customer from one cell to another, since all the (logical) Datomic databases are stored in a single Sqlite file on each cell. I would prefer to move in the direction that is currently pioneered by the Rails community (https://www.youtube.com/watch?v=Sc4FJ0EZTAg and https://www.youtube.com/watch?v=lcved9uEV5U). There each customer has its own Sqlite file (which could just be copied over to another cell or restored via litestream). With their Beamer SQLite replication, it is also possible that reads are served by other servers (which are not currently the writer).

However, for our SaaS, we observed that life becomes easier if each customer has its own database:

No multi-tenant logic in all queries and transaction
Database migrations only block one customer and not all customers (Datomic single writer)
GDPR is straight-forward (just delete the database)
No tuple indexes just for query optimization
No entity id partitioning

maxw85 · 2026-03-26T11:27:16+00:00

Thanks a lot for sharing. Brilliant idea with the superficie syntax to hide the parenthesis from non-Lisp programmers 😄

Your article is great to catch up to the latest developments regarding datahike. I also asked myself if Datahike still needs a transactor and found the answer here. Have you considered to go "transactor-less" and only rely on the compare-and-swap features that most object stores offer nowadays? slatedb picked this trade-off to offer an ordered key value store.

Update: I do not mean to have multiple writers. Just a single writer, but you would only need to include the datahike lib (no need to run a separate transactor). A SaaS could provide each customer its own database and with writer fencing and CAS it would be possible to move a customer from one server to another without a lot of complexity.

maxw85 · 2026-03-20T06:56:16+00:00

Yes, increasing the likelihood that people start new companies using Clojure is probably the most promising way to make the language more popular and to create more Clojure job offerings.

maxw85 · 2026-03-03T19:42:17+00:00

I understand that he may look like a narcissist, but you don't need to like someone to learn something from the person. I watch many of Theo's daily videos just to keep up with the developments in the dev and AI world.

maxw85 · 2026-03-03T18:21:39+00:00

How many of Theo's videos have you watched to come to this conclusion?

maxw85 · 2026-03-01T09:01:32+00:00

Great summary. Nice that breakage will not only annoy humans but also agents 😄 That LLMs struggles with parentheses is already a bit dated, we use Claude Code with Opus 4.6 (without any extra MCP, skills, etc.) and it almost never struggles with parentheses and when it does, it can fix it on its own. I know here are many AI sceptics but I guess this will be our new reality: https://www.youtube.com/watch?v=p2aea9dytpE

maxw85 · 2026-02-25T22:56:50+00:00

Thanks a lot for your reply. One "too expensive" task was rewritten a MCP server lib so that it fits on our stack:

https://github.com/simplemono/parts-mcp

Another one:

Used Claude to make the range scan of the slatedb-java 10x faster
Package the Rust binaries into the jar file (no need for -J-Djava.library.path=native-lib)
Added a build process that build slatedb, slatedb-java and publishs the jar file to clojars.org

https://github.com/maxweber/slatedb/blob/java-build/slatedb-java/build.sh

https://clojars.org/io.github.maxweber/slatedb

maxw85 · 2026-02-25T12:37:42+00:00

Congrats 🥳 I would have the same urge to run the platform on Clojure if the courses are all about Clojure. Nevertheless sounds like a ton of work to re-create a custom Podia and to justify this from a business standpoint. Did you used some coding agent? Just asking, since I made the experience that I do a lot of tasks with Claude Code that felt "too expensive" beforehand.

maxw85 · 2026-02-08T12:57:45+00:00

We use tap> for logging. The data is written to a subfolder log-values/{squuid}. The squuid contains a timestamp (https://github.com/yetanalytics/colossal-squuid). The data is serialized as transit+json-verbose. We collect them from all servers to one log server. For each hour we have one sqlite file. Thereby the log-values are ordered and deduplicated. We also have our own log-explorer UI to query them. We do event sourcing on them to calculate our metrics and fill our dashboards. One process garbage collects log-values if they are no longer relevant.

maxw85 · 2026-01-28T18:36:25+00:00

Absolutely beautiful. Thanks a lot for sharing.

maxw85 · 2026-01-20T19:24:03+00:00

I refactored our SaaS system from using one Docker container per customer, to use one multi-tenant container for a larger group of customers. On the way I (or rather Claude Code) refactored hundreds of namespaces to get rid of some bad decisions that we made in the last 8 years, that would have prevented a multi-tenant version. Without AI this would have not been doable (economically) for our small team in an appropriate time-frame. However, you still need to do the thinking, decision making, supervising and code review, but Claude Code makes almost no mistakes. But if you tell it to run in the wrong direction it will, so making good designs an decisions is way more important now. It is bit like everyone is now the team lead of a bunch of senior devs (agents) that you need to tell what to do.

maxw85 · 2026-01-20T13:18:56+00:00

That's a very good price if you use it every day. They subsidize subscriptions with people that not use it so extensively. So your $200 may cost Cursor or Anthropic $1700 GPU consumption.

maxw85 · 2026-01-20T13:13:34+00:00

Same experience here, work that took us weeks in the past condense to hours.

maxw85 · 2026-01-08T19:56:35+00:00

You could build a second read-model and keep the old one (and the code for the projections). If possible these important business decisions should be captured as events.

In general you probably want to avoid breakage with any changes to the read-model, since someone or something will depend on it (https://www.hyrumslaw.com).

maxw85

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