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Architecting for Latency

Dan Pritchett, Technical Fellow at eBay, spoke about "Architecting for Latency". His aim was not to talk about minimizing latency, as you might expect, but rather to architect as though you believe latency is unavoidable and real.

We all know the effect latency can have on performance. That's the first-level effect. If you consider synchronous systems---such as SOAs or replicated DR systems---then latency has a dramatic effect on scalability as well. Whenever a synchronous call reaches across a long wire, the latency gets added directly to the processing time.

For example, if client A calls service B, then A's processing time will be at least the sum of B's processing time, plus the latency between A and B. (Yes, it seems obvious when you state it like that, but many architectures still act as though latency is zero.)

Furthermore, latency over IP networks is fundamentally variable. That means A's performance is unpredictable, and can never be made predictable.

Latency also introduces semantic problems. A replicated database will always have some discrepancy with the master database. A functionally or horizontally partitioned system will either allow discrepancies or must serialize traffic and give up scaling. You can imagine that eBay is much more interested in scaling than serializing traffic.

For example, when a new item is posted to eBay, it does not immediately show up in the search results. The ItemNode service posts a message that eventually causes the item to show up in search results. Admittedly, this is kept to a very short period of time, but still, the item will reach different data centers at different times. So, the search service inside the nearest data center will get the item before the search service inside the farthest. I suspect many eBay users would be shocked, and probably outraged, to hear that shoppers see different search results depending on where they are.

Now, the search service is designed to get consistent within a limited amount of time---for that item. With a constant flow of items, being posted from all over the country, you can imagine that there is a continuous variance among the search services. Like the quantum foam, however, this is near-impossible to observe. One user cannot see it, because a user gets pinned to a single data center. It would take multiple users, searching in the right category, with synchronized clocks, taking snapshots of the results to even observe that the discrepancies happen. And even then, they would only have a chance of seeing it, not a certainty. 

Another example. Dan talked about payment transfer from one user to another. In the traditional model, that would look something like this.

Synchronous write to both databases

You can think of the two databases as being either shards that contain different users or tables that record different parts of the transaction.

This is a design that pretends latency doesn't exist. In other words, it subscribes to Fallacy #2 of the Fallacies of Distributed Computing. Performance and scalability will suffer here.

(Frankly, it has an availability problem, too, because the availability of the payment service Pr(Payment) will now be Pr(Database 1) * Pr(Network 1) * Pr(Database 2) * Pr(Network 2). In other words, the availability of the payment service is coupled to the availability of Database 1, Database 2, and the two networks connecting Payment to the two databases.)

Instead, Dan recommends a design more like this:

Payment service with back end reconciliation

In this case, the payment service can set an expectation with the user that the money will be credited within some number of minutes. The availability and performance of the payment service is now independent from that of Database 2 and the reconciliation process. Reconciliation happens in the back end. It can be message-driven, batch-driven, or whatever. The main point is that it is decoupled in time and space from the payment service. Now the databases can exist in the same data center or on separate sides of the globe. Either way, the performance, availability, and scalability characteristics of the payment service don't change. That's architecting for latency.

Instead of ACID semantics, we think about BASE semantics. (A cute retronym for Basically Available Soft-state Eventually-consistent.) 

Now, many analysts, developers, and business users will object to the loss of global consistency. We heard a spirited debate last night between Dan and Nati Shalom, founder and CTO of GigaSpaces about that very subject.

I have two arguments of my own to support architecting for latency.

First, any page you show to a user represents a point-in-time snapshot. It can always be inaccurate even by the time you finish generating the page. Think about a commerce site saying "Ships in 2 - 3 days". That's true at the instant when the data is fetched. Ten milliseconds later, it might not be true. By the time you finish generating the page and the user's browser finishes rendering it (and fetching the 29 JavaScript files needed for the whizzy AJAX interface) the data is already a few seconds old. So global consistency is kind of useless in that case, isn't it? Besides, I can guarantee there's already a large amount of latency in the data path from inventory tracking to the availability field in the commerce database anyway.

Second, the cost of global consistency is global serialization. If you assume a certain volume of traffic you must support, the cost of a globally consistent solution will be a multiple of the cost of a latency-tolerant solution. That's because global consistency can only be achieved by making a single master system of record. When you try to reach large scales, that master system of record is going to be hideously expensive.

Latency is simply an immutable fact of the universe. If we architect for it, we can use it to our advantage. If we ignore it, we will instead be its victims.

For more of Dan's thinking about latency, see his article on InfoQ.com

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