Print connection strings after creating the cluster.

Author: FGasper

Dockerfile

@@ -21,4 +21,6 @@ ENV PATH=$PATH:/root/.local/bin
 
 COPY entrypoint.sh .
 
+COPY print_connstrs.py .
+
 ENTRYPOINT ["./entrypoint.sh"]

entrypoint.sh

@@ -14,5 +14,8 @@ shift
 echo "Starting MongoDB cluster …"
 mlaunch "$@" --bind_ip_all
 
+echo
+./print_connstrs.py < data/.mlaunch_startup
+
 # Hang forever:
 tail -f /dev/null

print_connstrs.py

@@ -0,0 +1,126 @@
+#!/usr/bin/env python3
+import json
+import sys
+from typing import List, Optional, Tuple
+
+
+def build_hosts(hostname: str, start_port: int, count: int) -> str:
+    """
+    Build host list like: hostname:start_port,hostname:start_port+1,...
+    Returns just the host list (no mongodb:// prefix).
+    """
+    return ",".join(f"{hostname}:{start_port + i}" for i in range(count))
+
+
+def is_numeric_string(s: str) -> bool:
+    """
+    Return True if s is a non-empty string of digits (e.g. "3", "10").
+    """
+    return isinstance(s, str) and s.isdigit()
+
+
+def parse_sharded(sharded_field) -> Tuple[bool, Optional[List[str]], Optional[int]]:
+    """
+    Parse the 'sharded' field.
+
+    Returns:
+        (is_sharded, shard_names, shard_count)
+
+        - Non-sharded (replset):
+          (False, None, None)
+
+        - Sharded, numeric-string single-element array (e.g. ["3"]):
+          (True, None, 3)  # shard_count = 3
+
+        - Sharded, named shards (e.g. ["shardA", "shardB"]):
+          (True, ["shardA", "shardB"], 2)
+
+    Raises:
+        ValueError if the 'sharded' field has an invalid shape.
+    """
+    if sharded_field is None:
+        return False, None, None
+
+    if not isinstance(sharded_field, list):
+        raise ValueError(f"Invalid 'sharded' value (not null or list): {sharded_field!r}")
+
+    if len(sharded_field) == 0:
+        raise ValueError("Invalid 'sharded' value: empty array")
+
+    # All members must be strings
+    if not all(isinstance(x, str) for x in sharded_field):
+        raise ValueError(
+            "Invalid 'sharded' value: list must contain only strings "
+            f"(got: {sharded_field!r})"
+        )
+
+    # Case 1: single numeric-string element → shard count
+    if len(sharded_field) == 1 and is_numeric_string(sharded_field[0]):
+        shard_count = int(sharded_field[0])
+        if shard_count <= 0:
+            raise ValueError(f"Invalid shard count in 'sharded': {sharded_field[0]!r}")
+        return True, None, shard_count
+
+    # Case 2: array of shard names (strings)
+    shard_names = sharded_field
+    shard_count = len(shard_names)
+    return True, shard_names, shard_count
+
+
+def main():
+    data = json.load(sys.stdin)
+    parsed_args = data.get("parsed_args", {})
+
+    hostname = parsed_args.get("hostname", "localhost")
+    base_port = int(parsed_args.get("port", 27017))
+    nodes = int(parsed_args.get("nodes", 1))
+    sharded_field = parsed_args.get("sharded")
+    mongos_count = int(parsed_args.get("mongos", 0))
+
+    try:
+        is_sharded, shard_names, shard_count = parse_sharded(sharded_field)
+    except ValueError as e:
+        print(f"Error parsing 'sharded': {e}", file=sys.stderr)
+        sys.exit(1)
+
+    # Non-sharded: replica set
+    if not is_sharded:
+        hosts = build_hosts(hostname, base_port, nodes)
+        conn_str = f"mongodb://{hosts}"
+        print("Connection string:")
+        print(conn_str)
+        return
+
+    # Sharded deployment
+
+    # 1. Mongos connection string
+    mongos_hosts = build_hosts(hostname, base_port, mongos_count)
+    mongos_conn_str = f"mongodb://{mongos_hosts}"
+    print("Main connection string:")
+    print(f"{mongos_conn_str}")
+
+    print()
+
+    # 2. Shards
+    # Shard ports start immediately after the last mongos port.
+    shard_base_port = base_port + mongos_count
+
+    # If shard_names is None, then shard_count came from numeric string like ["3"]
+    if shard_names is None:
+        width = max(2, len(str(shard_count)))
+        shard_names = [f"shard{(i + 1):0{width}d}" for i in range(shard_count)]
+
+    # Sanity: shard_count should match len(shard_names)
+    if shard_count is None:
+        shard_count = len(shard_names)
+
+    print("Per-shard connection strings:")
+    for i, shard_name in enumerate(shard_names):
+        first_node_port = shard_base_port + i * nodes
+        shard_hosts = build_hosts(hostname, first_node_port, nodes)
+        shard_conn_str = f"mongodb://{shard_hosts}"
+        print(f"{shard_name}: {shard_conn_str}")
+
+
+if __name__ == "__main__":
+    main()