PuppetDB 4.4: Debugging with remote REPL

Included in Puppet Enterprise 2017.2.

PuppetDB includes a remote REPL interface, which is disabled by default.

This interface is mostly of use to developers who know Clojure and are familiar with PuppetDB’s codebase. It allows you to modify PuppetDB’s code on the fly. Most users will never need to use the REPL, and for security reasons, it should generally be left disabled.

Enabling the REPL

To enable the REPL, you must edit PuppetDB’s config file to enable it, configure the listening IP address, and choose a port:

# /etc/puppetdb/conf.d/repl.ini
[nrepl]
enabled = true
port = 8082
host = 127.0.0.1

After configuration, restart the PuppetDB service.

Connecting to a remote REPL

Once PuppetDB is accepting remote REPL connections, you can connect to it and begin issuing low-level debugging commands and Clojure code.

For example, with a NREPL configured on port 8082, and using Leiningen to connect:

# lein repl :connect localhost:8082
Connecting to nREPL at localhost:8082
REPL-y 0.3.1
Clojure 1.6.0
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e

user=> (+ 1 2 3)
6

Executing functions

Within the REPL, you can interactively execute PuppetDB’s functions. For example, to manually compact the database:

user=> (use 'puppetlabs.puppetdb.cli.services)
nil
user=> (use 'puppetlabs.puppetdb.scf.storage)
nil
user=> (use 'clojure.java.jdbc)
nil
user=> (garbage-collect! (:database configuration))
(0)

Redefining functions

You can also manipulate the running PuppetDB instance by redefining functions on the fly. Let’s say that for debugging purposes, you’d like to log every time a catalog is deleted. You can just redefine the existing delete-catalog! function dynamically:

user=> (ns puppetlabs.puppetdb.scf.storage)
nil
puppetlabs.puppetdb.scf.storage=>
(def original-delete-catalog! delete-catalog!)
#'puppetlabs.puppetdb.scf.storage/original-delete-catalog!
puppetlabs.puppetdb.scf.storage=>
(defn delete-catalog!
  [catalog-hash]
  (log/info (str "Deleting catalog " catalog-hash))
  (original-delete-catalog! catalog-hash))
#'puppetlabs.puppetdb.scf.storage/delete-catalog!

Now any time that function is called, you’ll see a message logged.

Note that any changes you make to the running system are transient; they don’t persist between restarts of the service. If you wish to make longer-lived changes to the code, consider running PuppetDB directly from source.

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