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By deciding to store the queue position in each servant, we give explicit knowledge about the queue to each servant, introducing a major interdependency into the design An alternative approach is to keep each servant's queue position in a separate data structure, such as a hash table that stores pairs of object ID and queue position This approach makes it possible to remove a servant's queue entry as part of etherealize instead of removing the entry in the servant's destructor Unfortunately, this approach still does not solve the problem that every operation invocation must somehow maintain the LRU order of the queue, so we still need support from the servant implementation Worse, by storing pairs of object ID and queue position in the hash table, we are essentially duplicating the Active Object Map that is already maintained by the POA This doubles the storage overhead for keeping track of servants For servants that contain only a small amount of state, this approach may well be too expensive The conclusion of this discussion is that although we can use the Evictor pattern with servant activators, we must work hard to fit the pattern to the implementation In turn, this suggests that it pays to plan ahead and to choose POA policies carefully Turning source code that relies on servant activators into source code that uses servant locators (or vice versa) is not easy, so making the correct decision up front is worthwhile 1267 Interactions with Collection Manager Operations The Evictor pattern offers a way to limit memory consumption in servers without compromising performance unduly However, to some degree, it interferes with collection manager operations, such as list and find For the implementation in 10, the implementation of list was trivial: we simply iterated over the list of servants and invoked the _this member function to create a list of references However, as soon as we do not have all servants in memory, we cannot do this because to invoke _this, we need a servant The solution is to not rely on servants being in memory at all Instead, we can use the make_dref function shown on page 546:
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CCS::Controller::ThermometerSeq * Controller_impl:: list() throw(CORBA::SystemException) { // Create a new thermometer sequence Because we know // the number of elements we will put onto the sequence, // we use the maximum constructor CCS::Controller::ThermometerSeq_var listv = new CCS::Controller::ThermometerSeq(m_assetssize()); listv->length(m_assetssize()); // Loop over the m_assets map and create a // reference for each device CORBA::ULong count = 0; AssetMap::iterator i; for (i = m_assetsbegin(); i != m_assetsend(); i++) listv[count++] = make_dref(m_poa, i->first);
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return listv_retn();
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This implementation avoids the need to instantiate a servant for each device just so that we can call _this The version shown here applies to our servant locator implementation For servant activators, we change only one line of code:
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// CORBA::ULong count = 0; AssetMap::iterator i; for (i = m_assetsbegin(); i != m_assetsend(); i++) listv[count++] = make_dref(m_poa, *i); //
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With a servant activator, the controller contains only a set of asset numbers instead of a map, so instead of passing i->first to make_dref, we pass *i These implementations are similar to the examples we use to introduce servant managers in 11 The Evictor pattern also interferes with the find operation The implementation in 10 iterates over the servants in memory to locate devices with matching attributes Because not all servants may be in memory, this approach does not work You may be tempted to write something like this instead:
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// // Loop over input list and look up each device CORBA::ULong listlen = slistlength(m_assetssize()); for (CORBA::ULong i = 0; i < listlen; i++) { AssetMap::iterator where; // // Iterator for asset map
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CCS::Controller::SearchCriterion sc = slist[i]key_d(); if (sc == CCS::Controller::LOCATION) { // Search for matching asset location for (where = m_assetsbegin(); where != m_assetsend(); where++) { Thermometer_var t = make_dref(m_poa, where->first); if (strcmp(t->location(), slist[i]keyloc()) == 0) // Found a match } // } //
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The strategy here is to create an object reference for each device and then to ask the device for its location This works, but it has a hideous flaw: this linear search causes
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