To configure a MongoDB replica set on Ubuntu, install the same official mongodb-org release on at least three reachable hosts, give every member a resolvable hostname, set the identical replica-set name in /etc/mongod.conf, configure net.bindIp for inter-member traffic, restart each daemon, and run rs.initiate() once from one member. Confirm the election with rs.status() and inspect the saved configuration with rs.conf().
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This guide covers a self-managed deployment on 64-bit Ubuntu 24.04 (Noble), 22.04 (Jammy), or 20.04 (Focal) using MongoDB’s official packages. The repository path and package instructions must match the MongoDB version, Ubuntu release, and CPU architecture you actually use.
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What a MongoDB replica set does
A replica set is a group of mongod processes that maintain copies of the same data and coordinate an election. One reachable member becomes the primary; the remaining members normally serve as secondaries and can become primary if an election is required. The configuration is shared by the set, but the daemons still run independently on their Ubuntu hosts.
A practical production layout uses three members on separate hosts. Separating hosts reduces the chance that one machine failure removes every voting member. A two-member arrangement can be configured, but it has less tolerance for a failure because the remaining member may not be able to form a majority. This guide uses three members named mongodb0.example.net, mongodb1.example.net, and mongodb2.example.net.
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Replica-set availability depends on more than installing three processes. The members must have consistent identity, a shared set name, working name resolution, and network paths in both directions. If any one of those assumptions is wrong, the service may appear to be running locally while the set remains unhealthy.
Before you install: requirements and decisions
| Item | Requirement or decision |
|---|---|
| Operating system | 64-bit Ubuntu 24.04, 22.04, or 20.04 for the documented MongoDB 8.0 package path; other MongoDB releases may support a different set of Ubuntu releases. |
| Packages | Use MongoDB’s official mongodb-org repository and packages. Ubuntu’s unrelated mongodb package conflicts with them. |
| Hosts | Prefer separate servers with stable, mutually resolvable DNS names. |
| Addressing | Every member must be able to resolve and connect to every other member on the configured MongoDB port, normally 27017. |
| Replica-set name | Choose one name, such as rs0, and use it identically on every daemon and in rs.initiate(). |
| Security | Do not expose an unauthenticated daemon on a public interface. Plan firewall restrictions and member authentication before allowing remote access. |
Choose hostnames, not changing IP addresses
Use names such as mongodb0.example.net:27017 in the member list. MongoDB recommends hostnames because an address change then does not require rewriting the replica-set configuration. From MongoDB 5.0 onward, nodes configured only with an IP address can fail startup validation, so hostname-based member addresses are especially important on newer releases.
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Plan the three-host layout
Place the three hosts so a single server or local failure does not remove all voting members. Separate physical or virtual machines are preferable to three processes on one Ubuntu host. Before installation, record each host’s intended DNS name, the MongoDB port, the Ubuntu release, and the MongoDB version. This simple inventory prevents one member from receiving a repository line intended for a different codename or release.
Choose the deployment boundary
Self-managed Ubuntu gives you direct control over operating-system placement, network paths, package timing, authentication files, and firewall rules. A hosted service such as MongoDB Atlas removes local installation and host maintenance, but places those operational decisions with the provider. The table below is a decision aid rather than a pricing comparison.
| Concern | Self-managed Ubuntu | Hosted deployment |
|---|---|---|
| Installation and patching | Your team installs repositories, packages, and operating-system updates. | The provider supplies the database environment and handles the underlying service. |
| Backups and recovery | You design, run, and verify backup and restore procedures. | Backup features and retention depend on the provider’s service configuration. |
| Network placement | You choose the hosts, subnets, firewall policy, and proximity to applications. | Placement is selected from the provider’s available regions and networking options. |
| Authentication | You configure member and client authentication and protect the key material. | The hosted platform supplies its own control plane and connection-security workflow. |
| Scaling | You provision and reconfigure members and their storage. | Scaling controls are supplied by the hosted service and its plans. |
| Cost control | You pay for servers, storage, traffic, administration, and maintenance. | Charges follow the provider’s selected cluster and service options. |
There is no single universal price comparison. Self-managed cost includes the hosts, storage, network traffic, operating-system work, monitoring, backup equipment or services, and staff time. A hosted plan converts much of that work into a service charge, but the exact amount depends on the provider’s selected cluster and options. Compare total operational ownership, not just the first server invoice.
Install MongoDB on every Ubuntu member
1. Confirm release and architecture
On each planned member, verify the Ubuntu release and that the machine is 64-bit before adding a repository. Do not copy a repository entry for one Ubuntu codename onto another. MongoDB publishes separate instructions by server release and Ubuntu release; select the path matching your target version and architecture.
Use the same intended MongoDB release on all members. Mixing package generations makes troubleshooting harder and can produce behavior that is unrelated to replica-set configuration. Write down the selected release before running package commands so that the same repository path is applied consistently on all three hosts.
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2. Add the matching official repository
Follow the version-specific MongoDB installation instructions to install the repository prerequisites, import the MongoDB signing key, create the matching /etc/apt/sources.list.d/mongodb-org-<version>.list file, and refresh package metadata:
sudo apt-get updateThe documented prerequisite tools include gnupg and curl. The exact key URL and repository line vary by MongoDB release and Ubuntu codename, so use the instructions for that combination rather than substituting a line from another release. Repeat the repository setup on every member; a repository configured on one host is not shared with the others.
3. Install the server packages
sudo apt-get install -y mongodb-orgRun the installation on every member. If Ubuntu’s unrelated mongodb package is installed, remove that conflicting package according to your change procedure before installing mongodb-org; the two package families are not interchangeable. Check the package result on each machine rather than assuming that a successful installation on one host means all hosts are identical.
4. Start and enable the daemon
sudo systemctl start mongod
sudo systemctl status mongod
sudo systemctl enable mongodstatus should show a running service. Starting and enabling are separate actions: start launches the current service, while enable arranges for it to start with the system. If systemd says that mongod.service is missing, reload unit files and retry:
sudo systemctl daemon-reload
sudo systemctl start mongodFor startup and runtime diagnostics, inspect the default log at /var/log/mongodb/mongod.log. Read the log when a service exits immediately, when a configuration edit is rejected, or when a peer remains unhealthy even though the local status command reports that the process is running.
Prepare the configuration on each member
Set the same replica-set name
Edit /etc/mongod.conf on every host. The replication.replSetName value must be identical everywhere. In this example it is rs0. The value is case-sensitive for practical configuration purposes, so do not use RS0 on one member and rs0 on another.
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Bind localhost and the member’s reachable hostname
MongoDB binds to localhost by default. That default permits local administration but prevents remote members from connecting. Add localhost and the hostname that other members and approved clients will use. Use the appropriate hostname on each machine:
replication:
replSetName: rs0
net:
bindIp: localhost,mongodb0.example.netOn the second and third hosts, replace mongodb0.example.net with that host’s own name. The names in this setting must correspond to addresses that the other members can resolve and reach. An equivalent command-line setting is --bind_ip, but keeping the value in the configuration file makes the service definition consistent.
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Do not treat bindIp as a firewall. It determines which local interfaces accept MongoDB connections, while firewall rules determine which remote sources can reach the port. Configure both: bind to the required local name or address, then restrict the port to trusted member and client networks.
Apply the edit
Restart the daemon on each host after changing /etc/mongod.conf:
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Initialize the replica set once
Connect locally to one member
Choose one member and open the MongoDB shell locally:
mongoshInitialization is a one-time replica-set operation. MongoDB explicitly requires running rs.initiate() on only one mongod instance. Do not paste the command into all three shells. Running it repeatedly or concurrently creates avoidable configuration errors.
Submit the member configuration
Use one member document for each host. The configuration identifier must match replication.replSetName:
rs.initiate({
_id: "rs0",
members: [
{ _id: 0, host: "mongodb0.example.net:27017" },
{ _id: 1, host: "mongodb1.example.net:27017" },
{ _id: 2, host: "mongodb2.example.net:27017" }
]
})The command starts the replica-set election process. The host strings must be the names and ports that every member can resolve and connect to, not names that work only from the shell where you typed the command. Include the port explicitly for every member, even when all members use the normal MongoDB port.
What success looks like
A successful initiation returns an acknowledgment rather than a request to initialize again. The member on which you ran the command may become primary, but the election result is not something to hard-code into your application configuration. Query the set to discover the current state. Elections can change after a failure or restart, so clients should use replica-set-aware connection behavior rather than assuming that the first host listed remains primary.
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Verify health and configuration
Inspect member state with rs.status()
rs.status()Run this after the election settles. Use it to identify the current primary, see which members are secondaries, and inspect health information for each member. A member that cannot resolve a peer, reach its port, or start with the expected configuration will show a problem here. Run the command again after correcting a network or configuration issue; the first output is only a snapshot of the election and health state.
Inspect the saved configuration with rs.conf()
rs.conf()Confirm that the replica-set name is rs0 in this example and that the host list contains all intended members with the correct ports. Comparing this output with the names in each /etc/mongod.conf file catches typos before they become a deployment mystery. The configuration shown by the shell is the set’s saved member list, while the YAML file controls each daemon’s local startup behavior; both need to agree on identity and connectivity.
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Once rs.status() identifies the primary, connect to that member for administrative operations that require primary state. If a different member wins the election, follow the status output rather than assuming that mongodb0.example.net is primary. Treat the primary as changeable and use the replica-set state to determine where a primary-only operation should run.
Secure a networked deployment
Restrict exposure before opening remote access
Never bind an unsecured MongoDB process to a public interface. Limit firewall rules to the required MongoDB port and trusted member or client sources. Ensure that every member can connect to every other member while untrusted internet hosts cannot. A reachable port without authentication is not a safe production configuration.
Configure internal member authentication
MongoDB’s keyfile procedure places the same security.keyFile on every member alongside the replica-set name and required bind addresses:
security:
keyFile: /etc/mongodb/keyfile
replication:
replSetName: rs0
net:
bindIp: localhost,mongodb0.example.netCopy the same keyfile securely to each host. Apply restrictive ownership and permissions suitable for the mongod service account, but confirm the actual account and package layout on the target Ubuntu installation before choosing commands. Restart all members after the file and configuration are in place, then reconnect clients using authentication. The exact ownership and permission commands are intentionally environment-dependent; a package-created service account and a custom service account may not be identical.
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Understand what a keyfile enables
Enabling a keyfile also enforces role-based access control for clients. Applications and administrators therefore need authenticated database users with appropriate roles; an unauthenticated shell workflow used during initial testing will no longer be sufficient. Plan the transition so that the required users and credentials are available before remote clients are switched to the secured configuration.
MongoDB documents keyfiles primarily for testing and development because their manageability and cryptographic strength are limited. For production, use X.509 certificates or another stronger enterprise authentication approach where your operating requirements support it. Whichever mechanism you choose, protect credentials and certificate material on every member and keep firewall access narrower than the set’s trusted network.
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Troubleshoot by symptom
| Symptom | Likely cause | Action |
|---|---|---|
mongod.service is not found |
systemd has not loaded the newly installed unit. | Run sudo systemctl daemon-reload, then start the service again and inspect its status. |
| A member cannot join | DNS, firewall policy, or net.bindIp prevents the peer connection. |
Check name resolution in both directions, permit the required member-to-member port, and ensure each daemon binds its reachable hostname rather than only localhost. |
rs.initiate() reports a configuration problem |
The replica-set names differ, a host entry is wrong, or initialization was attempted more than once. | Compare replSetName on every host with the _id in the command, verify every host and port, and run initialization only on the one intended member. |
| Startup validation fails after using addresses | A newer MongoDB release rejects an IP-only member configuration. | Replace member addresses with mutually resolvable hostnames, especially on MongoDB 5.0 and later, then restart and review the log. |
| The service starts but peers remain unhealthy | The process is running, but the advertised hostname is not reachable from another member. | Use rs.conf() to find the advertised value, correct DNS or the bind list, and check the firewall path before rechecking rs.status(). |
| Package installation reports conflicts | Ubuntu’s separate mongodb package is installed. |
Remove the conflicting package through your normal change procedure and install the official mongodb-org package instead. |
| Only local connections work | The daemon is still bound to localhost or the hostname is not included in the bind list. | Update net.bindIp with the member’s reachable hostname, restart mongod, and verify that firewall policy permits trusted peers. |
| The set has no primary after initialization | Members cannot communicate well enough to complete an election or do not form a majority. | Check every member’s status, host spelling, DNS resolution, bind address, and firewall path. Correct connectivity on all members, then inspect rs.status() again. |
Operational checklist
- All members run the same intended MongoDB package release and compatible Ubuntu architecture.
- Each host has a stable DNS name that all other members can resolve.
- Every daemon uses the same
replication.replSetName. net.bindIpcontains localhost and the member address used by peers and approved clients.- The MongoDB port is reachable between trusted members and blocked for untrusted sources.
rs.initiate()was executed once, with one entry for each member.rs.status()shows a primary and healthy secondaries.rs.conf()contains the expected name, hostnames, and ports.- Authentication is enabled before exposing the deployment beyond a controlled network.
- If a keyfile is used, the identical file is protected on every member and the service account can read it.
- The default log at
/var/log/mongodb/mongod.logis part of your first-line diagnostic process. - Any repository or package change is repeated consistently on every host.
How to decide whether this setup fits
Choose self-managed Ubuntu when you need direct control of host location, operating-system configuration, network policy, or package timing and you have a team responsible for patching, backups, monitoring, and recovery. Choose a hosted deployment when reducing local installation and infrastructure administration is more important than controlling those layers directly. In either case, the replica-set principles remain: members need consistent identity, reliable connectivity, a shared set name, and authentication appropriate to the deployment’s exposure.
Frequently Asked Questions
How many Ubuntu servers should a MongoDB replica set use?
A three-member layout on separate hosts is the practical pattern used in this guide. Two members can be configured, but a remaining single member may not be able to form a majority after one failure.
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Can I use IP addresses instead of hostnames?
Use stable, mutually resolvable hostnames. MongoDB recommends them, and from MongoDB 5.0 onward a node configured only with an IP address can fail startup validation.
Why can the members not connect even though mongod is running?
MongoDB binds to localhost by default. Check that each member’s net.bindIp includes its reachable hostname, that DNS works in both directions, and that firewall rules allow trusted traffic to the MongoDB port.
Where do I run rs.initiate()?
Run it once from a shell connected locally to one member. MongoDB explicitly says to run rs.initiate() on only one mongod instance.
How do I find the current primary?
Connect to a member and run rs.status(). The output identifies the current primary and the members serving as secondaries; do not assume the first host listed remains primary.
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rs.status() reports current health and member state, including the primary and secondaries. rs.conf() displays the saved replica-set name and host list so you can check identity and ports.
Is a MongoDB keyfile suitable for production?
MongoDB documents keyfiles mainly for testing and development because manageability and cryptographic strength are limited. For production, prefer X.509 certificates or another stronger enterprise authentication approach where appropriate.
What happens when I enable a keyfile?
The same keyfile must be available to every member, and enabling it also enforces role-based access control for clients. Applications and administrators must therefore connect with authenticated users and suitable roles.
Why does Ubuntu report a package conflict?
Ubuntu’s unrelated mongodb package conflicts with MongoDB’s official mongodb-org package. Remove the conflicting package through your normal change process before installing the official packages.
The Bottom Line
A reliable Ubuntu replica set is built from three consistent foundations: stable hostnames that every member can resolve, identical replication and bind settings on each daemon, and a one-time initialization followed by status and configuration checks. Keep the MongoDB port restricted, authenticate every networked deployment, and use stronger production authentication than a development keyfile when your requirements call for it.
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