When it comes to defining a strategy for web API security, OAG (Oracle API Gateway) and OES (Oracle Entitlements Server) together present a very interesting choice and are a very powerful combination indeed.
In this post we're going to take a look at what each component brings in (the skin) and then get our hands on actually describing the integration in detail (the guts).
OAG is designed to inspect and act on various types of messages that are delivered to it or just pass through it. It's usually positioned to be deployed on the DMZ (the De-Militarized Zone) within corporate networks. As such, it can block malicious traffic, authenticate users with a variety of protocols, integrate with anti-virus products, perform message throttling, thus delivering only the good stuff to your intranet servers and also off-loading them, decisively contributing to achieve some IT operational SLAs. More than that, OAG can switch protocols and transform messages. For instance, an organization may have SOAP-based web services and want to expose them as REST without any re-writing. Or implement SAML federation without touching origin systems. Or talk Kerberos or OAuth with clients and speak SAML with back-end servers. Or use it as an FTP server so that incoming files are immediately sent to a processing pipeline. The possibilities are numerous. Having mentioned these few features and examples, it's not unreasonable to think deploying OAG inside intranets. And that's not unusual, actually. It is a nice bridge with obvious benefits.
OES is designed to provide fine-grained authorization with externalized policies to client applications. It takes the coding of access decisions away from developers. Besides the obvious security pro, it shortens the change cycle, when a new security policy needs to be deployed. You simply avoid going through all the phases required for re-deploying your application just because of that change. It's true the new policy needs testing, but that's nowhere near when compared to what it takes to re-deploy a new application version. The time to market is drastically reduced. Now to the fine-grained part. OES can take a bunch of aspects in consideration when authorizing: the user identity, user roles, user attributes, context information about the request being made (like originating IP address), factors external to the request (like time of day, day of week, etc) and, of course, request data. Those combined makes it a very powerful authorization engine. It's not coincidence that OES is the component behind OAM's (Oracle Access Manager) authorization engine.
While OAG itself brings in authorization capabilities, in this field OES offers a much richer model. And if the organization already employs OES elsewhere, integrating it with OAG makes a lot of sense, because we end up with a single and consistent approach for authorization across applications.