How m Caching Works for Available MongoDB Versions: Complete Technical Guide

The m version manager caches the MongoDB version metadata JSON—not the actual binaries—to avoid repeated network requests, using the M_CACHE, M_CACHE_SRC, and M_CACHE_EXPIRY environment variables to control caching behavior.

The m tool by Aaron Heckmann is a lightweight MongoDB version manager implemented as a Bash script. Unlike version managers that cache binary artifacts, m implements an opt-in caching mechanism specifically for the version list metadata retrieved from https://downloads.mongodb.org/full.json. This design minimizes network overhead when querying available versions without consuming disk space for redundant binary storage.

How m Caches MongoDB Version Metadata

The caching logic resides entirely within the core script bin/m. Rather than caching MongoDB binaries themselves, the tool caches the JSON response containing all available versions, release channels, and download URLs.

Cache Configuration Environment Variables

Three shell variables control the caching behavior:

• M_CACHE – Enables or disables caching. Defaults to 0 (disabled).
• M_CACHE_SRC – Specifies the cache file path. Defaults to $M_DIR/cache-src.json.
• M_CACHE_EXPIRY – Defines cache validity in seconds. Defaults to 3600 (1 hour).

These variables are initialized at lines 32-43 of bin/m:

# Use cache?

CACHE=${M_CACHE:-0}

# Location of cached source file

CACHE_SRC=${M_CACHE_SRC:-$M_DIR/cache-src.json}

# Cache source expiry (seconds)

CACHE_EXPIRY=${M_CACHE_EXPIRY:-3600}

Reading the Cached Version List

When the get_all_versions function executes, it first checks if caching is enabled and whether the cached file is fresh. The script uses the file_age_in_seconds helper function to validate cache freshness against CACHE_EXPIRY.

At lines 14-22 in bin/m, the logic validates cache existence and age:

if [ $CACHE == 1 ]; then
  if [ -e $CACHE_SRC ]; then
    if [ $(file_age_in_seconds $CACHE_SRC) -lt $CACHE_EXPIRY ]; then
      all_versions=`cat $CACHE_SRC`
    fi
  fi
fi

If the cache exists and its age is less than the expiry threshold, m reads the JSON directly from disk instead of fetching from the remote endpoint.

Writing and Refreshing the Cache

When the cache is disabled, missing, or expired, m fetches fresh data from $src_url (pointing to https://downloads.mongodb.org/full.json). If caching is enabled, the newly fetched data persists to $CACHE_SRC for subsequent calls.

Lines 24-31 of bin/m implement the write logic:

if [ -z "$all_versions" ]; then
  all_versions=`$GET $src_url`
  if [ $CACHE ]; then
    echo "$all_versions" > $CACHE_SRC
  fi
fi

All version-querying commands—including list_versions, display_latest_version, and display_latest_stable_version—invoke get_all_versions, ensuring consistent cache utilization across the tool.

Practical Cache Configuration Examples

Configure m caching behavior using environment variables before executing commands.

Enable caching for a single session with the default 1-hour expiry:

export M_CACHE=1
m ls 4.4

Extend cache validity to 12 hours:

export M_CACHE=1
export M_CACHE_EXPIRY=$((12*60*60))
m ls 5.0

Force a fresh fetch by disabling the cache:

M_CACHE=0 m ls 6.0

Manually clear the cache file:

rm "$HOME/.local/m/cache-src.json"

Summary

• m caches only metadata, not MongoDB binaries, storing the JSON from downloads.mongodb.org/full.json locally.
• Caching is opt-in via the M_CACHE environment variable, which defaults to 0 (disabled).
• Cache location defaults to $M_DIR/cache-src.json but is configurable via M_CACHE_SRC.
• Default expiry is 3600 seconds (1 hour), controlled by M_CACHE_EXPIRY.
• Implementation resides in bin/m, specifically within the get_all_versions function and supporting cache validation logic.

Frequently Asked Questions

Does m cache MongoDB binaries or just version metadata?

m caches only the version metadata JSON file, not the actual MongoDB binaries. The cache stores the response from https://downloads.mongodb.org/full.json, which contains version numbers and download URLs, while the binaries themselves are downloaded on-demand during installation.

How do I enable caching in m?

Set the M_CACHE environment variable to 1 before running m commands. By default, M_CACHE is 0, meaning the tool fetches fresh version data from the MongoDB downloads API on every invocation unless caching is explicitly enabled.

Where does m store its cache file?

By default, m stores the cache at $M_DIR/cache-src.json, typically resolving to $HOME/.local/m/cache-src.json. You can override this location by setting the M_CACHE_SRC environment variable to a custom file path.

How long does m keep the version cache?

The default cache expiry is 3600 seconds (1 hour), defined by the M_CACHE_EXPIRY environment variable. After this period, m considers the cache stale and fetches fresh metadata from the remote endpoint upon the next version query.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}