Ably vs PubNub
Discover how PubNub compares to Ably, and understand which is right for your use case, based on dimensions such as core features, pricing, reliability, and scalability.
What is Ably?
Ably is a realtime infrastructure platform that provides APIs and SDKs to power live experiences at global scale, with no infrastructure to build or maintain. Ably’s products include:
- Pub/Sub: Managed WebSocket messaging at scale, with guaranteed ordering, exactly-once delivery, and connection recovery out of the box
- Chat, Spaces, and LiveSync: Purpose-built SDKs for common realtime use cases
- AI Transport: Resumable streaming and session continuity for AI applications
What is PubNub?
PubNub is a developer API platform that powers the realtime infrastructure in apps to build engaging Virtual Spaces where online communities can connect. It can serve as the foundation for online chat, live events, geolocation, remote IoT control, and realtime updates.
Compare Ably and PubNub
Ably and PubNub offer similar developer-facing primitives: channels, presence, and a publish/subscribe API. The differences emerge in production: when a connection drops, when a burst of traffic overflows a buffer, or when an AI application has to keep a token stream alive. The quick answers below cover the most common evaluation questions, and the full feature comparison table follows after them.
Common evaluation questions
How do Ably and PubNub handle message ordering and delivery guarantees?
Both PubNub and Ably guarantee message ordering, though the guarantees are scoped differently. PubNub does this via a monotonically increasing timetoken assigned centrally at the server, guaranteeing one ordered sequence per channel across all publishers. Ably does this via an incrementing serial, scoped per publisher per channel.
In practice, most applications publish from a single source per channel, so the difference rarely matters. Where the platforms genuinely diverge is exactly-once delivery. PubNub has no server-side deduplication, so a retried publish always creates a second, distinct message. Ably deduplicates natively, with no application code required.
The delivery guarantees deep dive covers ordering, exactly-once delivery, and reconnection in full.
What happens to messages on each platform when a client disconnects and reconnects?
On PubNub, messages published while a client is disconnected are queued in a buffer and replayed automatically on reconnect, but only within limits: up to 100 messages or 16 minutes, whichever comes first, discarding the oldest once that fills. Recovering a longer gap means manually replaying from Message Persistence using the timetoken from before the gap.
On Ably, every message a client missed is replayed automatically, in order and exactly once, as long as it reconnects within two minutes, with no configuration required. Beyond that window, its History API covers the plan’s retention period instead.
See the delivery guarantees deep dive for the full comparison of reconnection and history behavior.
What do Ably and PubNub guarantee for uptime, and what have they delivered?
PubNub and Ably both headline a 99.999% uptime SLA, and both reserve it for their upper tiers — PubNub from its Pro plan, Ably on Enterprise. Both also document a multi-region network with automatic failover, so neither the guarantee nor the architecture separates them.
What does differentiate the two platforms is the gap between guaranteed and delivered: an SLA is a commitment with a service credit attached, defining what you’re owed when uptime slips rather than what the platform has held. Ably has run seven consecutive years without a global outage. PubNub publishes no equivalent multi-year record.
For infrastructure and failover mechanics, see the delivery guarantees deep dive.
Which platform is better suited for AI streaming applications?
PubNub’s 32 KiB message cap forces chunking for longer model responses. Its buffer-based recovery doesn’t guarantee a dropped token stream comes back, and there’s no purpose-built session-continuity layer for AI use cases.
Ably closes each of those gaps directly: guaranteed ordered delivery, a larger message size, and two-minute automatic session recovery. Ably AI Transport packages the three into a single session layer, built for resumable token streaming.
The AI applications deep dive covers this in full.
What are Ably and PubNub’s message size and throughput limits?
PubNub caps every message at 32 KiB, flat across all plan tiers, with no paid upgrade path to a higher ceiling. Ably’s default message size is 64 KiB on Free and Standard, rising to 256 KiB on Pro and Enterprise. Throughput works the other way around: PubNub doesn’t cap publish throughput by plan, while Ably’s throughput ceiling scales with your paid tier.
The delivery guarantees deep dive explains what this means for chunking and batching in practice.
Feature comparison table
Let’s compare Ably and PubNub, looking at key dimensions such as their core features, pricing, integrations & interoperability, quality of service, performance & availability and security & compliance.
| Core features | |||
Pub/Sub messaging | Reduces communication code complexity, simplifying the process of building highly functional and architecturally complex realtime apps. | Ably | PubNub Yes |
Chat capabilities | Accelerates the time to implement rich chat experiences with features such as read receipts, typing indicators, and more. | Ably Yes Ably Chat supports 1:1, group messaging and live streaming chat, with APIs for edit & delete, message and room reactions, moderation, typing indicators, moderation and more. Learn more about Ably Chat | PubNub Yes |
Collaboration capabilities | Enables you to quickly integrate realtime collaborative features like live cursors, member location, avatar stacks, and component locking. | Ably Yes The Spaces SDK provides an opinionated set of abstractions for collaborative environments including live cursors, avatar stacks and component locking. Learn more | PubNub Partial PubNub's collaboration capabilities are largely powered by its Presence feature. There is no dedicated collaboration SDK. |
State sync capabilities | Enables realtime data synchronization across devices and users, ensuring a cohesive and up-to-date user experience. | Ably Yes Ably provides a self-hostable and managed database connector to stream updates over our serverless WebSocket platform. LiveSync provides seamless connector interactions for Postgres and MongoDB. Learn more | PubNub Partial |
Presence | Maintaining a view of which users are connected, and their associated metadata, enables their online status to be updated in realtime. | Ably Yes Ably’s presence API automatically syncs the full membership state on channel attach, ensuring clients always have an up-to-date and consistent view of who’s online. Learn more | PubNub Yes |
Occupancy | High-level metrics about the clients currently connected to a channel make it simple to show things such as connected user count, or display which channels are the most popular. | Ably Yes Ably’s Occupancy API delivers detailed, real-time metrics (connections, publishers, subscribers, presenceMembers, etc.) for every channel as part of its built-in monitoring capabilities. Learn more | PubNub Partial No dedicated Occupancy API, but occupancy counts are available via the Here Now API and Presence webhooks. |
Message interactions | Enables interaction with previously-sent messages, facilitating the implementation of features like message reactions and threads. | Ably Yes Ably’s annotations API lets you attach, update and aggregate metadata (reactions, flags, tags) to previously-sent messages and query the roll-up summary efficiently. Learn more | PubNub Yes PubNub’s Message Actions API allows metadata to be added to messages (reactions, receipts) but is tied to message persistence being enabled and uses a separate message-action endpoint rather than fully integrated mutation of the original message record. |
Message history | Enables clients to catch up on missed messages when inactive, ensuring a user doesn’t miss any important messages. | Ably Yes | PubNub Yes |
Push notifications | Cross-platform push notifications make it possible to deliver important and timely messages to users even when they’re inactive. | Ably Yes Support for Android and iOS push notifications. Improved web-push for browsers currently in progress. | PubNub Yes |
Message delta compression | Minimizes bandwidth and can reduce latency, particularly in scenarios where continuous updates are sent. | Ably Yes The delta option sends only the change (via VCDIFF) from the previous message on a channel. See Deltas docs | PubNub No |
Programmatic management | Enables the automation of provisioning, management, and testing of service resources, simplifying integration with existing development workflows such as CI. | Ably Yes Control API and a Terraform provider manage apps, keys, and channel rules as infrastructure. See Control API docs | PubNub Yes |
Server-side batching | Reduces the overall message count, lowers costs, and mitigates the risk of hitting rate limits during high-throughput scenarios. | Ably Yes Messages within a configurable interval group into one batch (up to 200 per batch), billed as one message. Not deduplicated by ID. See Batching docs | PubNub No |
Maximum message size | A message size limit below your payload size means custom chunking on both publisher and subscriber, plus manual ordering logic — significant engineering overhead for LLM responses, document content, or structured data. | Ably 64 KiB on Free and Standard; 256 KiB on Pro and Enterprise. | PubNub 32 KiB, flat across all plans. |
AI streaming support | Without purpose-built AI streaming infrastructure, ordering, chunking, and session recovery have to be built and maintained across the stack - and they tend to fail at the moments users notice most. | Ably Yes Ably AI Transport provides resumable token streaming, multi-device session continuity, guaranteed ordered delivery, and two-minute session recovery for AI applications. | |
| Pricing | |||
Free plan | With a free plan, you can test the service’s functionality and compatibility with your project before committing to a paid plan. | Ably Yes 6 million monthly messages, 200 concurrent connections, 200 concurrent channels, 500 messages per second. No credit card required. | PubNub Yes 200 MAU, or 1M transactions/Function executions, or 1GB storage (7 days), whichever comes first. No credit card required. |
Pricing model | The pricing model should align with your project's expected load, usage patterns, and budget in order to be cost-effective and efficient. | Ably Ably offers usage-based pricing (messages, connection minutes, and channel minutes) or MAU-based pricing, letting you choose the model that best fits your traffic pattern. | PubNub PubNub's pricing model is based on monthly active users (MAUs). |
Pricing plans | The pricing plans offered by cost. | Ably The free and paid plans are for per-minute and MAU models:
| PubNub Starter: $98/month/MAU: Up to 1000 MAUs, and up to 3000 transactions per MAU. Pro packages: Custom pricing Overages apply:
Their support packages, separate from their standard plans, also include an Enterprise option. |
| Integrations & interoperability | |||
SDKs | Supporting multiple languages and platforms offers greater flexibility when building cross-platform realtime apps. | Ably
| PubNub
Source: SDKs |
Supported realtime protocols | Support for multiple protocols provides the flexibility to choose a protocol that best suits your project’s requirements. | Ably
| PubNub
PubNub uses HTTP as the transport for their client libraries. A WebSocket compliant interface is provided in some libraries, however this is just a wrapper around an underlying HTTP transport. |
Serverless functions | Enables integration with third-party cloud providers by facilitating the execution of custom code against messages to perform business logic like on-the-fly translation. | Ably
| PubNub PubNub operates its own serverless architecture through its feature called PubNub Functions. |
Streaming & queueing | Provides a dependable method to reroute messages from the service to third-party streams and queues for further processing. | Ably Egress:
Ingress:
The Database Connector is designed for any database in the SQL-family, but at the moment only supports Postgres. Please contact us if you want support in other databases. | PubNub Egress:
Ingress:
|
Observability services | Enables realtime monitoring and troubleshooting by offering insights into service behavior directly in your observability platform of choice. | Ably Yes | PubNub Yes But access limits apply. Datadog dashboards are Platinum-Support-plan only; New Relic runs through PubNub's legacy BLOCKS product and needs your own paid New Relic account. |
CI/CD tools | Makes it possible to provision and configure service infrastructure as part of a CI or CD pipeline, enabling repeatable and reliable deployments. | Ably
| PubNub PubNub provides a Command Line Interface (CLI) specifically for managing and deploying PubNub Functions. |
| Quality of Service | |||
Proven scalability | Scalability is vital as it ensures the service can handle increased data load or users without compromising performance. | Ably Yes 30B+ monthly connections opened, backed by 7+ years since the last global outage. | PubNub Yes 25M+ Concurrent Connections |
Guaranteed message delivery | Ensures messages are never lost during transmission, even in the presence of network disruptions. | Ably Yes Guaranteed within a 2-minute reconnect window (full replay, in order). Beyond that, recoverable via History for the plan's retention period. See History docs | PubNub No guaranteed message delivery; queues up to 100 messages upon disconnection. |
Guaranteed message ordering | Maintains the sequence of messages as they were sent. This is particularly important in apps where the chronological order of messages is essential for meaningful communication. | Ably Yes Per publisher per channel - via message serials.:ctaSee message ordering docs: | PubNub Yes Channel-wide — one ordered sequence per channel, via server-assigned timetokens. |
Exactly-once message delivery | Guarantees that each message is processed exactly once, preventing data inconsistencies that can arise from duplicate processing or missing messages. | Ably Yes | PubNub No |
Connection state recovery | Without connection state recovery, filling a disconnection gap requires a custom message store, fetch-on-reconnect logic, and deduplication - engineering the platform should handle natively. | Ably Yes If a client reconnects within two minutes, all missed messages are replayed in order with exactly-once semantics. No custom catch-up logic required. Beyond that, History covers 1 day (Free) up to unlimited (Enterprise). | PubNub Yes PubNub supports connection state recovery and has an automatic reconnection process to handle brief network interruptions. PubNub message delivery page |
| Performance & availability | |||
Uptime Guarantee | An uptime guarantee instills confidence in the reliability of the service and protects your business from the negative impacts of downtime. | Ably Yes 99.999% SLA on Enterprise, SLO elsewhere. 7+ years of 100% uptime. | PubNub Yes Up to 99.999% from Pro; no SLA on Free or Starter. |
Global edge network | By bringing servers (Points of Presence, or PoP) geographically closer to the devices of end users, and routing requests to the nearest PoP, global latency is reduced to a minimum. | Ably Yes 11+ core regions, 700+ edge points of presence. | PubNub Yes |
Multi-region data replication (message durability) | By replicating data across multiple regions, the risk of data loss or downtime is greatly mitigated since if data is lost or a server fails in one region, the information can be retrieved from another. | Ably Yes Messages replicate across 3+ physically isolated locations before acknowledgment; 99.999999% survivability. See the Ably architecture overview | PubNub Yes PubNub supports multi-region data replication across different points of presence except for file sharing. Read more |
No single point of failure or congestion | Having no single point of failure means a system is resilient and can continue to operate even if one part fails. Avoiding a single point of congestion ensures messages flow efficiently across the system and avoids bottlenecks that could lead to performance issues under load. | Ably Yes | PubNub Yes |
Low latency | Low latency is crucial for realtime apps as it ensures swift and efficient data transmissions, providing a smoother and more responsive user experience. | Ably Yes
| PubNub <100ms on paid (Starter/Pro) plans; PubNub's free-plan target is closer to <250ms. |
| Security & compliance | |||
API key authentication | Simplifies the authentication code on trusted servers compared to requesting, managing, and refreshing tokens. | Ably Yes | PubNub Yes |
Token-based authentication | Provides a means to securely authenticate user devices against your user management system. | Ably Yes | PubNub Yes |
Single Sign-On (SSO) authentication | SSO streamlines login processes, boosts security by minimizing password use, and meets compliance needs for secure data access management. | Ably Yes Google/GitHub sign-in on every plan. Full SAML SSO with any identity provider, plus SCIM, is Enterprise-only. See more in our SSO docs | PubNub Yes Pro plan only. Setup requires your own IT team to configure the identity provider. |
Rules for permissions and operations | Provides control over which users can subscribe and publish to certain channels. | Ably Yes Capabilities scoped per key or token control subscribe, publish, and presence access down to channel-name patterns. | PubNub Yes PubNub supports rules for permissions and operations through its Access Manager. See Access Manager docs |
End-to-end encryption | Ensures that the data transmitted between the client and the API server remains confidential and secure while in transit. | Ably Yes TLS by default for all traffic. True end-to-end encryption (AES-256, opt-in per channel) is available via the cipher option. Learn more about Ably's encryption | PubNub |
Encryption at rest | Ensures data stored by the service is secure and compliant, while also mitigating the risks of a data breach. | Ably Yes | PubNub Yes |
Compliance | Compliance with regulations can impact your ability to meet legal obligations in your industry. | Ably
Ably also offers EU and US-only data storage. | PubNub PubNub is compliant with five policies for the security and privacy of its services:
|
Disclaimer: This comparison was created based on documentation and resources freely available online about Ably and PubNub. The content was last updated on 10 Jul 2026 for Ably and on 12 Aug 2026 for PubNub. Be sure to double-check everything before you make any decisions. If you do find anything incorrect or out of date, then please let us know.
Where Ably and PubNub diverge in production
These three scenarios show where the differences surface, not during integration, but at the moments that test the infrastructure. They cover a reconnecting client, a burst that outruns a buffer, and an AI response interrupted mid-stream.
Chat application with reconnecting mobile clients
A commuter’s phone drops signal for three minutes mid-conversation.
On PubNub, the default subscriber buffer holds up to 100 messages for up to 16 minutes, so a short gap like this is usually covered automatically on reconnect. A longer or higher-volume gap can overflow the buffer, and the oldest messages are discarded first.
On Ably, this gap is longer than the two-minute automatic recovery window, so the client falls back to Ably’s History API to retrieve everything it missed, rather than the automatic in-order replay a shorter gap would have triggered.
A live sports app during a scoring spike
A live sports app publishes score and event updates to thousands of subscribers. A goal sends a burst of updates at once, and a viewer on a patchy mobile connection drops out just as it lands.
On PubNub, the subscriber buffer holds up to 100 messages, and a goal-burst can fill that faster than the gap closes. Once it overflows, the oldest updates are discarded — silently, not reordered, just gone.
Recovering them means the application has to detect the gap itself and replay from Message Persistence.
On Ably, a client reconnecting inside the two-minute window gets everything it missed, in order per publisher per channel. Beyond that, the same updates stay recoverable through History rather than lost.
An AI response interrupted mid-stream
A user’s Wi-Fi drops for a few seconds while an AI assistant is streaming a response, then they pick up the conversation on a different device.
On PubNub, a drop this short usually falls inside the subscriber buffer, so the tokens replay on reconnect. The gap is the handoff: the buffer belongs to that connection, so a different device starts cold.
Picking the stream up there means replaying from Message Persistence with your own offset tracking.
With Ably AI Transport, the client resumes the stream from the point of disconnection, and because the session lives on an Ably channel rather than a single connection, the new device can pick it up in the same place.
How each platform handles AI and LLM streaming
Both platforms support publish/subscribe messaging. For LLM token streaming, the gaps in PubNub’s default behavior become the things your team has to build around.
| Constraint | PubNub | Ably |
|---|---|---|
| Message size | 32 KiB flat cap, the same on every plan tier. Larger model responses need chunking. | 64 KiB default, rising to 256 KiB on Pro and Enterprise. Full responses delivered natively. |
| Delivery guarantee | Channel-wide ordering via timetoken. Delivery is at-most-once by default, with no server-side deduplication — a retry from your code arrives twice. | Exactly-once with guaranteed ordering, including across a reconnect. |
| Session recovery | Automatic buffer replay for up to 16 minutes (100 messages, oldest discarded first). Anything beyond that needs a manual replay from Message Persistence using timetokens. | Two-minute automatic replay, plus persisted History for the plan’s retention period. |
Ably AI Transport provides guaranteed delivery, automatic session recovery, and message sizes large enough for typical model responses as a single drop-in layer.
When to use each platform
| PubNub is appropriate when… | Ably is a better fit when… |
|---|---|
| Occasional message loss during a buffer overflow or extended disconnect is an acceptable experience, for presence, dispatch, or geolocation feeds. | Every message must arrive in order and be delivered exactly once, including after a reconnect. |
| You’re comfortable building custom replay logic for gaps beyond PubNub’s 16-minute, 100-message buffer window. | Clients need to catch up automatically after a disconnection, without your team building custom replay logic. |
| Your payloads stay under PubNub’s 32 KiB cap. | Payloads regularly exceed 32 KiB. |
| You need native SDKs for IoT and embedded platforms (FreeRTOS, Mbed, POSIX C/C++), or built-in geolocation as a platform feature. | You’re building AI-powered products that need ordered token delivery and session continuity out of the box. |
| A published SLA is enough on its own, without a track record showing it’s been held. | You need an SLA that’s been demonstrably held over years, not just committed to. |
PubNub is a capable platform for workloads where occasional message loss is tolerable. Applications that need guaranteed exactly-once delivery, automatic catch-up after longer disconnections, or the larger message sizes and session continuity that AI streaming needs run into PubNub’s buffer and delivery-model limits sooner than teams expect.
The two platforms put responsibility in different places. PubNub guarantees ordering by default, but leaves deduplication and catch-up beyond its 16-minute buffer for your application to build. Ably handles ordering, deduplication, and reconnection at the infrastructure level, all by default. If occasional message loss and building your own replay logic are acceptable trade-offs for your use case, PubNub’s model may fit well. If they’re not, that’s the gap Ably’s guarantees are built to close.
Start building on Ably for free, or talk to the team about your requirements.
Hear from companies gaining their realtime advantage with AblyAbly immediately solved our issues around the user experience and scale. It supports our customers’ stringent expectations as well as future product development.”
Max Freiert
Product Group Lead
Ably copes easily with big loads. We don’t have to worry about stability, even when we get huge traffic spikes.”
Johan Bengtsson
CTO
Ably delivers all the performance and reliability we need as well as taking our compliance and encryption requirements in its stride.”
Jeremy Hutchings
CTO
We’re not just benefiting from Ably’s ability to elasticity scale, but also from the freedom their SDKs give us to innovate.”
Chad Larter
Senior Director, Technical Operations
The core infrastructure for delivering live sports data to our customers is Ably. We sleep easier at night knowing it can be relied on for consistent low latency and message integrity at scale.”
Gary Williams
IT Infrastructure Team Lead
Ably gives us the reliable, low-latency AI transport we need for Messenger and Fin. No polling, no dropped messages, just a platform we can finally build next-generation AI experiences on.”
Colin Kennedy
Principal Product Engineer, Fin
Rock solid reliability — in terms of both uptime and customer connectivity.”
Brad Ward
Co-founder and CTO, TeamRetro
Try Ably for free to discover the benefits for yourself
Ably has built reliable realtime infrastructure so you don’t have to. On our free plan you benefit from:
- 6M monthly messages
- 200 concurrent channels
- 200 concurrent connections
