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A WebSocket pipeline for live Pakistan Stock Exchange (PSX) prices should run only on a feed you are licensed to use, and its reliability depends far more on feed-tier selection, gap detection and recovery than on the socket itself. Settle the rights question first, choose the feed tier by the event granularity you need, and keep the socket reader isolated from slower work. No PSX publication gives an end-to-end latency figure for a WebSocket pipeline, so any latency number you publish has to come from your own measurements, reported against a definition you state.
Start with rights, because they shape the architecture
PSX’s legal notice on its Data Services & Vending page prohibits dissemination, transmission, sale and commercial use of its market data, on a live or delayed basis, “without acquiring respective rights/license from the PSX.” The covered material includes stock prices, bids and asks, volumes, index levels and PUCARS corporate announcements, and the notice names websites and applications among the channels it applies to. A price that is visible on a public page is therefore not, by that fact alone, permission to republish it.
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The same page adds a narrower point that matters even for internal tools: “Any content and data available on the PSX website are for information purpose only and its commercial usage (whether in the form of internal consumption or third-party sale as derived data) is strictly prohibited unless acquired with prior approval of PSX.”
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteBefore you choose a feed, decide which of these describes the product you are building:
#1 Best Overall
- Internal consumption, such as a dashboard used only inside your organization.
- Public display, such as a website or mobile app that shows prices to visitors.
- Derived data, such as indices, alerts or analytics sold to others.
- Redistribution, meaning live data passed on to sub-clients or third-party applications.
Each category can require different rights. Record which one you are building and send that description to PSX’s Market Data Team, which handles licensing and authorization. The same page lists local and international data vendors, but a vendor’s authorization does not remove the need for a PSX license and permission for third-party or sub-client use. A multi-tenant product therefore needs that permission confirmed separately from the vendor contract.
Choose the feed tier by the granularity you need
PSX lists three real-time tiers: Real Time Data Feed Level 1, Level 1+ and Level 2. It also offers end-of-day data, historical data and PUCARS announcements, which matter if you do not need live events. The real-time tiers differ in two ways that drive design: how much of the order book you receive, and whether you receive every tick.
| Tier | What PSX lists | Order-book content | Tick-by-tick data |
|---|---|---|---|
| Real Time Data Feed Level 1 | Level 1 services (contents not itemized in PSX’s tier description) | Not itemized in PSX’s tier description | Not listed for this tier |
| Level 1+ | Level 1 services plus top-ten order-book facilities | MBO best ten bid and ask rates and quantities; MBP top ten volume records grouped by price level | Not listed for this tier |
| Level 2 | Level 1+ services plus tick-by-tick data | Level 1+ order-book facilities | Included |
Two design consequences follow. If your product only shows last prices, check whether Level 1 covers that need before paying for more depth. If you must reconstruct an order book or account for every tick, Level 1+ is the minimum to examine, and Level 2 is the only listed tier that includes tick-by-tick events. The tier also sets how much data your queues must absorb, so choose it before you size anything.
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The PSX FIX specification and RFC 6455 establish the building blocks below. The layout itself is an engineering recommendation, not a PSX-published reference architecture, and each component must be checked against the vendor’s actual message schema.
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Ingest from a server-side process
Open the connection from a server process using the authentication the vendor documents. Keep credentials out of browsers, client bundles, logs and metric labels. Confirm whether the vendor requires a secure WebSocket endpoint and which authentication scheme it supports, since these details vary by provider.
Normalize and validate every message
Decode each message into an internal event that records the symbol, the event type, the source timestamp if the vendor supplies one, a receive timestamp taken at your edge, the source identity, and the sequence value if one is supplied. Reject events that lack required fields and count the rejects, because a rising reject rate is an early sign of a schema change. Keep raw payloads for a bounded diagnostic window, and only where your contract and license permit storage.
Keep the socket reader isolated
The reader should do little more than receive frames, timestamp them and hand them to a bounded queue. Database writes, dashboards and slow consumers run behind that queue. When a queue fills, decide the overload behavior in advance and log which one ran. For a last-price display, coalescing to the latest state per symbol may be acceptable. Trade or order-book events that a downstream use case depends on must never be dropped silently. Queue sizes should come from measured load; PSX’s published materials do not prescribe them.
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Secure the keys and alert on the right signals
Store keys in a secret manager, restrict network access and restrict who can read raw market data. Rotate credentials under the provider’s policy. Alert on authentication failures, reconnect loops, sequence gaps, stale data, queue saturation, parse failures and clock anomalies. Retention and audit rules come from your data license, not from your logging defaults.
Rank #3
Detecting missing messages
A connection that stays open does not prove that every message arrived. Continuity has to be checked in the application, and the method depends on whether your vendor exposes sequence identifiers.
When the feed carries sequence numbers
PSX’s FIX specification describes message sequence numbers as the way messages are identified, and it makes the receiving application responsible for detecting gaps and requesting a resend. If your vendor’s WebSocket messages carry an equivalent field, apply the same rule separately for each stream. The logic below is illustrative and is not a vendor-specified algorithm:
expected = last_seq[stream] + 1
if seq == expected:
apply(event); last_seq[stream] = seq
elif seq < expected:
discard as duplicate; compare content with the stored event
else:
gap = seq - expected
mark stream stale; request resend or snapshot
The resend mechanism is documented for PSX’s FIX specification. Whether a given WebSocket feed offers resend is a separate question, and you should not assume it does.
When the feed has no sequence numbers
Without a sequence field, the stream alone cannot tell you which messages were lost during an outage. Ask whether the vendor sends a full snapshot on reconnect. If it does, rebuild state from that snapshot and discard what you held before the outage. If it does not, treat every affected stream as stale from the moment the socket dropped until the next full state message arrives, and show users that the interval has a gap rather than presenting the series as complete.
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Liveness is not freshness
RFC 6455 defines WebSocket as a protocol layered over TCP that begins with an HTTP Upgrade handshake, and it defines Ping and Pong control frames alongside text and binary data frames. Its keepalive rule is strict: “Upon receipt of a Ping frame, an endpoint MUST send a Pong frame in response, unless it already received a Close frame.” That keeps the connection honest, but it says nothing about whether market data is still arriving. A socket can answer every Ping and still carry no prices at all.
Track two clocks: the socket state, and the time of the last accepted application event for each stream. Staleness thresholds belong to each stream and session rather than to the whole connection, because a thinly traded symbol can be quiet for long stretches without anything being wrong. Set each threshold from the observed behavior of that symbol under the tier you bought, and alert when a stream exceeds it.
Recovery after a disconnect
PSX’s published materials do not establish one universal WebSocket reconnect or replay procedure. Treat the sequence below as the design pattern to implement, and confirm each step with your vendor.
- On socket close, mark every subscribed stream stale and record its last accepted sequence value and its last application event time.
- Reconnect with bounded exponential backoff and random jitter, so that a vendor outage does not turn into a synchronized retry storm.
- Reauthenticate using the documented scheme. Treat repeated authentication failures as an alert rather than an endless retry loop.
- Resubscribe to the same symbols and feed tier. If the vendor confirms subscriptions, check each confirmation against what you requested.
- Request a replay from the last accepted sequence value, or fetch a snapshot, wherever the vendor provides one.
- Apply the recovered events in sequence order using the continuity rule above, and discard duplicates.
- Clear the stale flag for each stream only after its reconciliation finishes, and log the gap interval for audit.
Provider examples and what does not transfer
Two public examples show how much WebSocket interfaces vary. Neither describes a PSX requirement.
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Capital Stake documentation
- Lists
/2.0/market/feed,/2.0/market/feed/fulland/2.0/market/feed/l2endpoints for its L0, L1 and L2 access. - Uses a bearer token for authentication.
- States that only two concurrent connections are allowed per authentication token.
That two-connection limit belongs to Capital Stake’s documentation. It is not a PSX-wide rule, and you should not assume it applies to another vendor or to a direct PSX arrangement. Confirm the production hostname, scope, limits, message schema and recovery support with the vendor itself.
pyPSX SDK documentation
- Describes a
LiveFeedclass that opens one/ws/marketconnection for multiple subscribed symbols. - States that the class handles reconnection.
This is third-party SDK documentation. It may suit a Python project as an implementation option, but it does not establish PSX endorsement, a licensed entitlement or a feed service-level agreement. Confirm that your license covers whatever data the SDK carries before you adopt it.
Measuring latency without an invented number
The PSX materials and the protocol standard cited here publish no end-to-end latency figure or guarantee for a WebSocket feed, and no independently sourced benchmark for this design exists. Any figure you publish must come from your own instrumentation, with its definition stated beside it. Three common definitions give different answers:
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- Exchange event time to receipt at your edge. This requires a source timestamp from the vendor and synchronized clocks; without them the figure cannot be measured.
- Receipt to normalized event, which isolates your parsing and validation cost.
- Receipt to consumer delivery, which includes queue delay and fan-out.
Use a monotonic clock for elapsed durations inside a process. Compare wall-clock timestamps across machines only after checking clock offset. Report distributions rather than averages, broken down by stream and symbol, and record queue delay separately so that a slow consumer is not mistaken for a network problem.
Quick Recap
Questions to put to a vendor before you build
- Written redistribution scope for your category, including how sub-clients are treated.
- Feed tier, symbol universe and market session coverage.
- Production WebSocket hostname, whether the endpoint is secure, and the supported authentication scheme.
- Connection and subscription limits per token, and what happens when a limit is exceeded.
- Message schema, including source timestamps, event types and any sequence identifier.
- Snapshot and replay behavior after a disconnect, and whether a resend can be requested.
- Failover and maintenance behavior.
- Latency and availability commitments, and whether they are stated in the contract.
- Geographic availability, support path and data retention rules.
- Price and contract term.
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